add link to plotly-geo to README

Author: emilykl

README.md

@@ -141,6 +141,8 @@ or conda
 conda install -c plotly plotly-geo=1.0.0
 ```
 
+`plotly-geo` can be found on Github at https://github.com/plotly/plotly-geo.
+
 ## Copyright and Licenses
 
 Code and documentation copyright 2019 Plotly, Inc.