Update README.md
added blurb on how to upgrade
@@ -25,7 +25,7 @@ generate an access token and access token secret. With these four variables
in hand you are ready to start using twarc.
1. install [Python](http://python.org/download) (2 or 3)
-2. pip install twarc
+2. pip install twarc (if upgrading, pip install twarc==version#, so pip install twarc==1.3.1)
## Quickstart:
