Add Documentation (#125)

Add sphinx docs with readthedocs integration.

Docs can be built locally with `poe docs`.

.github/CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contributing
+
+There's lots to do, and we're working hard, so any help is welcome!
+
+- :speech_balloon: Join us on [Slack](https://join.slack.com/t/betterproto/shared_invite/zt-f0n0uolx-iN8gBNrkPxtKHTLpG3o1OQ)!
+
+What can you do?
+
+- :+1: Vote on [issues](https://github.com/danielgtaylor/python-betterproto/issues).
+- :speech_balloon: Give feedback on [Pull Requests](https://github.com/danielgtaylor/python-betterproto/pulls) and [Issues](https://github.com/danielgtaylor/python-betterproto/issues):
+    - Suggestions
+    - Express approval
+    - Raise concerns
+- :small_red_triangle: Create an issue:
+    - File a bug (please check its not a duplicate)
+    - Propose an enhancement
+- :white_check_mark: Create a PR:
+    - [Creating a failing test-case](https://github.com/danielgtaylor/python-betterproto/blob/master/betterproto/tests/README.md) to make bug-fixing easier
+    - Fix any of the open issues
+        - [Good first issues](https://github.com/danielgtaylor/python-betterproto/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22)
+        - [Issues with tests](https://github.com/danielgtaylor/python-betterproto/issues?q=is%3Aissue+is%3Aopen+label%3A%22has+test%22)
+    - New bugfix or idea
+        - If you'd like to discuss your idea first, join us on Slack!

.github/workflows/code-quality.yml

@@ -9,8 +9,8 @@ on:
       - '**'
 
 jobs:
-  black:
-    name: Black
+  check-formatting:
+    name: Check code/doc formatting
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
@@ -18,3 +18,9 @@ jobs:
       uses: lgeiger/black-action@master
       with:
         args: --check src/ tests/
+
+    - name: Install rST dependcies
+      run: python -m pip install doc8
+    - name: Lint documentation for errors
+      run: python -m doc8 docs --max-line-length 88 --ignore-path-errors "docs/migrating.rst;D001"
+      # it has a table which is longer than 88 characters long