add a documentation badge and update pypi hyperlink
![image](https://user-images.githubusercontent.com/7570107/85285114-3969ac00-b488-11ea-88ff-5fca1b34af1f.png)
-![](https://img.shields.io/github/v/release/moj-analytical-services/splink?include_prereleases)
+[![pypi](https://img.shields.io/github/v/release/moj-analytical-services/splink?include_prereleases)](https://pypi.org/project/splink/#history)
[![Downloads](https://pepy.tech/badge/splink/month)](https://pepy.tech/project/splink)
+[![Documentation](https://img.shields.io/badge/API-documentation-blue)](https://moj-analytical-services.github.io/splink/)
# Fast, accurate and scalable probabilistic data linkage using your choice of SQL backend.
