Add more info to CONTRIBUTING.md (#892)

This adds more general information, along with an important warning
about AI-generated code.

Author: mhucka

CONTRIBUTING.md

@@ -1,29 +1,123 @@
-# How to Contribute
+# How to contribute
 
-We'd love to accept your patches and contributions to this project. There are
-just a few small guidelines you need to follow.
+We'd love to accept your patches and contributions to this project. We do have
+some guidelines to follow, covered in this document, but don't be concerned
+about getting everything right the first time! Create a pull request (discussed
+below) and we'll nudge you in the right direction.
 
-## Contributor License Agreement
+## Before you begin
 
-Contributions to this project must be accompanied by a Contributor License
-Agreement (CLA). You (or your employer) retain the copyright to your
-contribution; this simply gives us permission to use and redistribute your
-contributions as part of the project. Head over to
-<https://cla.developers.google.com/> to see your current agreements on file or
-to sign a new one.
+### Sign our Contributor License Agreement
 
-You generally only need to submit a CLA once, so if you've already submitted one
-(even if it was for a different project), you probably don't need to do it
-again.
+Contributions to this project must be accompanied by a [Contributor License
+Agreement](https://cla.developers.google.com/about) (CLA). You (or your
+employer) retain the copyright to your contribution; the CLA simply gives us
+permission to use and redistribute your contributions as part of the project.
+Please visit https://cla.developers.google.com/ to see your current agreements
+on file or to sign a new one. You generally only need to submit a Google CLA
+once, so if you've already submitted one (even if it was for a different
+project), you probably don't need to do it again.
 
-## Code Reviews
+> [!WARNING]
+> Please note carefully clauses [#5](https://cla.developers.google.com/about/google-corporate#:~:text=You%20represent%20that%20each%20of%20Your%20Contributions%20is%20Your%20original%20creation)
+> and [#7](https://cla.developers.google.com/about/google-corporate#:~:text=Should%20You%20wish%20to%20submit%20work%20that%20is%20not%20Your%20original%20creation%2C%20You%20may%20submit%20it%20to%20Google%20separately)
+> in the CLA. Any code that you contribute to this project must be **your**
+> original creation. Code generated by artificial intelligence tools **does
+> not** qualify as your original creation.
+
+### Review our community guidelines
+
+We have a [code of conduct](CODE_OF_CONDUCT.md) to make the Stim project an open
+and welcoming community environment. Please make sure to read and abide by the
+code of conduct.
+
+## Contribution process
 
 All submissions, including submissions by project members, require review. We
-use GitHub pull requests for this purpose. Consult
-[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
-information on using pull requests.
+use the tools provided by GitHub for [pull
+requests](https://help.github.com/articles/about-pull-requests/) for this
+purpose. The preferred manner for submitting pull requests is to fork the Stim
+[repository](https://github.com/quantumlib/Stim), create a [git
+branch](https://git-scm.com/book/en/v2/Git-Branching-Branches-in-a-Nutshell) in
+this fork to do your work, and when ready, create a pull request from this
+branch to the main Stim repository. The subsections below describe the process
+in more detail.
+
+Pleae make sure to follow the [Google Style
+Guides](https://google.github.io/styleguide/) in your code, particularly the
+[style guide for Python](https://google.github.io/styleguide/pyguide.html).
+
+### Repository forks
+
+1.  Fork the Stim repository (you can use the _Fork_ button in upper right
+    corner of the [repository page](https://github.com/quantumlib/Stim)).
+    Forking creates a new GitHub repo at the location
+    `https://github.com/USERNAME/Stim`, where `USERNAME` is your GitHub user
+    name.
+
+1.  Clone (using `git clone`) or otherwise download your forked repository to
+    your local computer, so that you have a local copy where you can do your
+    development work using your preferred editor and development tools.
+
+1.  Check out the `main` branch and create a new git branch from `main`:
+
+    ```shell
+    git checkout main -b YOUR_BRANCH_NAME
+    ```
+
+    where `YOUR_BRANCH_NAME` is the name of your new branch.
+
+### Development and testing
+
+Do your work and `git commit` your changes to your branch as needed.
+
+We use several tools to test code and perform other activities such as checking
+formatting against the style guidelines. You can run those tools locally during
+development. Wrapper scripts are located in the [`check/`](./check/)
+subdirectory to simplify running the tools.
+
+*   Run `check/pytest` to run the Pytest suite
+*   Run `check/mypy` to run the Mypy type checker
+*   Run `check/pylint` to run the Pylint code linter
+
+### Pull requests and code reviews
+
+1.  If your local copy has drifted out of sync with the `main` branch of the
+    main Stim repository, you may need to merge the latest changes into
+    your branch. To do this, first update your local `main` and then merge your
+    local `main` into your branch:
+
+    ```shell
+    # Track the upstream repo (if your local repo hasn't):
+    git remote add upstream https://github.com/quantumlib/Stim.git
+
+    # Update your local main.
+    git fetch upstream
+    git checkout main
+    git merge upstream/main
+    # Merge local main into your branch.
+    git checkout YOUR_BRANCH_NAME
+    git merge main
+    ```
+
+    If git reports conflicts during one or both of these merge processes, you
+    may need to [resolve the merge conflicts](
+    https://docs.github.com/articles/about-merge-conflicts) before continuing.
+
+1.  Finally, push your changes to your fork of the Stim repo on GitHub:
+
+    ```shell
+    git push origin YOUR_BRANCH_NAME
+    ```
 
-## Community Guidelines
+1.  Now when you navigate to the Stim repository on GitHub
+    (https://github.com/quantumlib/Stim), you should see the option to create a
+    new pull request from your forked repository. Alternatively, you can create
+    the pull request by navigating to the "Pull requests" tab near the top of
+    the page, and selecting the appropriate branches.
 
-This project follows
-[Google's Open Source Community Guidelines](https://opensource.google/conduct/).
+1.  A reviewer from the Stim team will comment on your code and may ask for
+    changes. You can perform the necessary changes locally, commit them to your
+    branch as usual, and then push changes to your fork on GitHub following the
+    same process as above. When you do that, GitHub will update the code in the
+    pull request automatically.