docs: Update CONTRIBUTING.md (#809)

Signed-off-by: Adam Gardner <[email protected]>

Author: agardnerIT

CONTRIBUTING.md

@@ -22,5 +22,90 @@ The project uses remote buf packages, changing the remote generation source will
 export GOPRIVATE=buf.build/gen/go
 ```
 
+## DCO Sign-Off
+
+A DCO (Developer Certificate of Origin) sign-off is a line placed at the end of
+a commit message containing a contributor's "signature." In adding this, the
+contributor certifies that they have the right to contribute the material in
+question.
+
+Here are the steps to sign your work:
+
+1. Verify the contribution in your commit complies with the
+   [terms of the DCO](https://developercertificate.org/).
+
+1. Add a line like the following to your commit message:
+
+   ```shell
+   Signed-off-by: Joe Smith <[email protected]>
+   ```
+
+   You MUST use your legal name -- handles or other pseudonyms are not
+   permitted.
+
+   While you could manually add DCO sign-off to every commit, there is an easier
+   way:
+
+    1. Configure your git client appropriately. This is one-time setup.
+
+       ```shell
+       git config user.name <legal name>
+       git config user.email <email address you use for GitHub>
+       ```
+
+       If you work on multiple projects that require a DCO sign-off, you can
+       configure your git client to use these settings globally instead of only
+       for Brigade:
+
+       ```shell
+       git config --global user.name <legal name>
+       git config --global user.email <email address you use for GitHub>
+       ```
+
+    1. Use the `--signoff` or `-s` (lowercase) flag when making each commit.
+       For example:
+
+       ```shell
+       git commit --message "<commit message>" --signoff
+       ```
+
+       If you ever make a commit and forget to use the `--signoff` flag, you
+       can amend your commit with this information before pushing:
+
+       ```shell
+       git commit --amend --signoff
+       ```
+
+    1. You can verify the above worked as expected using `git log`. Your latest
+       commit should look similar to this one:
+
+       ```shell
+       Author: Joe Smith <[email protected]>
+       Date:   Thu Feb 2 11:41:15 2018 -0800
+
+           Update README
+
+           Signed-off-by: Joe Smith <[email protected]>
+       ```
+
+       Notice the `Author` and `Signed-off-by` lines match. If they do not, the
+       PR will be rejected by the automated DCO check.
+
+## Conventional PR Titles
+
+When raising PRs, please format according to [conventional commit standards](https://www.conventionalcommits.org/en/v1.0.0/#summary)
+
+For example: `docs: some PR title here...`
+
 Thanks!
 Issues and pull requests following these guidelines are welcome.
+
+## Markdown Lint and Markdown Lint Fix
+
+PRs are expected to conform to markdown lint rules.
+
+Therefore, run `make markdownlint-fix` to auto-fix _most_ issues.
+Then commit the results.
+
+For those issues that cannot be auto-fixed, run `make markdownlint`
+then manually fix whatever it warns about.