Merge #578

578: Add docs.rs link to README r=michaelkirk a=dabreegster

- [X] I agree to follow the project's [code of conduct](https://github.com/georust/geo/blob/master/CODE_OF_CONDUCT.md).
- [ ] I added an entry to `CHANGES.md` if knowledge of this change could be valuable to users.

No changelog entry added

Co-authored-by: Dustin Carlino <dabreegster@gmail.com>

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://travis-ci.org/georust/geo.svg?branch=master)](https://travis-ci.org/georust/geo)
 [![geo on Crates.io](https://meritbadge.herokuapp.com/geo)](https://crates.io/crates/geo)
 [![Coverage Status](https://coveralls.io/repos/github/georust/geo/badge.svg?branch=trying)](https://coveralls.io/github/georust/geo?branch=trying)
+[![Documentation](https://docs.rs/geo/badge.svg)](https://docs.rs/geo)
 
 # geo