Add notes about specific release branches.

INSTALL

@@ -51,6 +51,13 @@ like to contribute an installation script, we would welcome it!)
 
         git clone git://github.com/mininet/mininet.git
 
+   Note that the above git command will check out the latest and greatest
+   Mininet (which we recommend!) If you want to run the last tagged/released
+   version of Mininet, use:
+
+        git clone git://github.com/mininet/mininet
+        git checkout -b 2.1.0 2.1.0
+
    If you are running Ubuntu, you may be able to use our handy
    `install.sh` script, which is in `mininet/util`.