Add Documentation (#125)

Add sphinx docs with readthedocs integration.

Docs can be built locally with `poe docs`.

pyproject.toml

@@ -33,6 +33,8 @@ pytest-asyncio = "^0.12.0"
 pytest-cov = "^2.9.0"
 pytest-mock = "^3.1.1"
 tox = "^3.15.1"
+sphinx = "3.1.2"
+sphinx-rtd-theme = "0.5.0"
 asv = "^0.4.2"
 
 [tool.poetry.scripts]
@@ -48,6 +50,7 @@ test        = { cmd = "pytest --cov src", help = "Run tests" }
 types       = { cmd = "mypy src --ignore-missing-imports", help = "Check types with mypy" }
 format      = { cmd = "black . --exclude tests/output_",   help = "Apply black formatting to source code" }
 clean       = { cmd = "rm -rf .coverage .mypy_cache .pytest_cache dist betterproto.egg-info **/__pycache__ tests/output_*", help = "Clean out generated files from the workspace" }
+docs        = { cmd = "sphinx-build docs docs/build", help = "Build the sphinx docs"}
 bench       = { shell = "asv run master^! && asv run HEAD^! && asv compare master HEAD", help = "Benchmark current commit vs. master branch"}
 
 # CI tasks