added instructions to install the release candidate (#105)

Excellent! Thanks very much @datapolitan!

Author: richarddunks

README.md

@@ -32,12 +32,19 @@ the internet into geospatial raster files. Bounding boxes can be passed in both
 
 ## Installation
 
-Python 3 only:
+**Python 3 only**
 
+[Latest released version](https://github.com/darribas/contextily/releases/tag/v0.99.0):
 ```sh
-pip3 install contextily
+pip3 install contextily # installs the latest released version (v0.99.0)
 ```
 
+Latest [release candidate](https://github.com/darribas/contextily/releases/tag/v1.0rc2) (includes functionality such as `add_basemap` coming in version 1.0:
+```sh
+pip3 install contextily==1.0rc2 # installs the latest release candidate (v1.0rc2) 
+```
+
+
 ## Contributors
 
 * [Dani Arribas-Bel](http://darribas.org/) ([`@darribas`](http://twitter.com/darribas))