update readme with doc links (#36)

README.md

@@ -2,6 +2,7 @@
 [![Coverage](https://img.shields.io/azure-devops/coverage/solararbiter/solarforecastarbiter/1/master.svg)](https://dev.azure.com/solararbiter/solarforecastarbiter/_build/latest?definitionId=1&branchName=master)
 [![Total alerts](https://img.shields.io/lgtm/alerts/g/SolarArbiter/solarforecastarbiter-core.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/SolarArbiter/solarforecastarbiter-core/alerts/)
 [![codecov](https://codecov.io/gh/solararbiter/solarforecastarbiter-core/branch/master/graph/badge.svg)](https://codecov.io/gh/solararbiter/solarforecastarbiter-core)
+[![Documentation Status](https://readthedocs.org/projects/solarforecastarbiter-core/badge/?version=latest)](https://solarforecastarbiter-core.readthedocs.io/en/latest/?badge=latest)
 
 # solarforecastarbiter-core
 Core Solar Forecast Arbiter data gathering, validation, processing, and
@@ -27,6 +28,9 @@ pytest solarforecastarbiter
 flake8 solarforecastarbiter
 ```
 
+# Documentation
+
+[solarforecastarbiter-core.readthedocs.io](https://solarforecastarbiter-core.readthedocs.io/en/latest/)
 
 # Architecture
 Sketch of how some SFA components interact, including the core: