Add contributing file

CONTRIBUTING.md

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+# Contributing to this repository
+
+Most of this repository is structured for **Sphinx**, a documentation engine built in Python.
+
+## Build the documentation
+
+The easiest way to build the documentation in this repository is to use `nox`, 
+a tool for quickly building environments and running commands within them.
+Nox ensures that your environment has all the dependencies needed to build the documentation.
+
+To do so, follow these steps:
+
+1. Install `nox`
+
+   ```
+   pip install nox
+   ```
+2. Build the documentation:
+
+   ```
+   nox -s docs_build
+   ```
+
+This should create a local environment in a `.nox` folder, build the documentation (as specified in the `noxfile.py` configuration), and the output will be in `_build/html`.
+
+To build live documentation that updates when you update local files, run the following command:
+
+```
+nox -s docs_live
+```