improve README

Author: mvexel

README.md

@@ -6,11 +6,21 @@ This is a thin wrapper around the OpenStreetMap [Overpass API](http://wiki.opens
 
 ## Install it
 
+### From PyPi
+
+This is the recommended way.
+
 ```bash
-$ python setup.py install
+pip install overpass
 ```
 
-The module is not on PyPi yet.
+### From source
+
+You will get the latest and greatest, but untested version.
+
+```bash
+$ python setup.py install
+```
 
 If you get an error similar to