doc: add description for using python2 with python3

README.md

@@ -13,7 +13,7 @@ Please refer to [The Black Belt Documentation](http://black-belt.readthedocs.org
 `virtualenv` assumed (`sudo pip install virtualenv`) and Python 2.7
 
 ```
-virtualenv venv
+virtualenv --python=/usr/local/bin/python2.7 venv
 source venv/bin/activate
 pip install paver
 pip install -r requirements-development.txt
@@ -28,6 +28,10 @@ paver test
 If you ran `bb init` and you want to do "discovery testing" with the integration tests,
 set `FORCE_DISCOVERY`environment variable to `1`.
 
+### Troubleshooting
+
+If you try run `paver bump` and get error `TypeError: 'map' object is not subscriptable` you can run `./venv/bin/paver bump` to fix this.
+
 ### Release
 
 When the time is right, run `paver bump`.