Update the ML2 README file with the latest tunnel changes

The ML2 README file was slightly out of date with all of the H2 changes
which went in around GRE and VXLAN tunneling. This patch updates it to
reflect what the code is capable of currently.

Fixes bug 1202743.

Change-Id: I578c1b62115f298c9de32d62161171fd2f3f5b7c
Merge "Update the ML2 README file with the latest tunnel changes"