CONTRIBUTING: Add doc

Author: Will Binns

CONTRIBUTING.md

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+# Welcome to developer.bitcoin.org's codebase
+
+Live site: [developer.bitcoin.org](https://developer.bitcoin.org)
+
+Report problems or help improve the site by opening a [new
+issue](https://github.com/bitcoin-dot-org/developer.bitcoin.org/issues) or [pull
+request](https://github.com/bitcoin-dot-org/developer.bitcoin.org/compare).
+
+## How to contribute
+
+This repo hosts the sources for the Bitcoin developer documentation.
+
+It is converted from Markdown to [reStructuredText
+(RST)](http://docutils.sourceforge.net/rst.html) and rendered with
+[Sphinx](http://www.sphinx-doc.org).
+
+### Render the documentation locally
+
+To render the documentation locally you first need to install Sphinx and the
+required theme modules, e.g. by running
+
+    pip install -r requirements.txt
+
+from the root of this repo. Then you can execute Sphinx by calling
+
+    make html
+
+This will generate HTML from the RST sources in the directory `_build/html`.
+It's all static HTML so you can just open the index.html file in your browser
+locally to view the rendered documentation.
+
+### Generation of RPC docs
+
+The documentation of the RPC commands is automatically generated from the help
+of a bitcoin client with a [helper
+tool](https://github.com/bitcoin-dot-org/developer.bitcoin.org/tree/master/helpers/rpc).
+This is the content in the [reference/rpc](reference/rpc) directory. Changes in
+these files need to be done through the helper tool or at least backported to
+the helper tool after doing them in this repo.
+
+## Code of Conduct
+
+Participation in this project is subject to a [Code of
+Conduct](https://github.com/bitcoin-dot-org/developer.bitcoin.org/blob/master/CODE_OF_CONDUCT.md).
+
+## Donations
+
+This project, developer.bitcoin.org, is community supported:
+[3FkenCiXpSLqD8L79intRNXUgjRoH9sjXa](bitcoin:3FkenCiXpSLqD8L79intRNXUgjRoH9sjXa)
+
+## Questions?
+
+Please contact Will Binns ([[email protected]](mailto:[email protected])) if you
+need help.