Add Documentation (#125)

Add sphinx docs with readthedocs integration.

Docs can be built locally with `poe docs`.

docs/index.rst

@@ -0,0 +1,33 @@
+Welcome to betterproto's documentation!
+=======================================
+
+betterproto is a protobuf compiler and interpreter. It improves the experience of using
+Protobuf and gRPC in Python, by generating readable, understandable, and idiomatic
+Python code, using modern language features.
+
+
+Features:
+~~~~~~~~~
+
+- Generated messages are both binary & JSON serializable
+- Messages use relevant python types, e.g. ``Enum``, ``datetime`` and ``timedelta``
+  objects
+- ``async``/``await`` support for gRPC Clients
+- Generates modern, readable, idiomatic python code
+
+Contents:
+~~~~~~~~~
+
+.. toctree::
+    :maxdepth: 2
+
+    quick-start
+    api
+    migrating
+
+
+If you still can't find what you're looking for, try in one of the following pages:
+
+* :ref:`genindex`
+* :ref:`modindex`
+* :ref:`search`