Upgrading to TG2.2 guidelines

[Alessandro Molina]

As many people are starting to try the new 2.2 set of releases I
thought it could be useful to point them to the upgrade guidelines.

The following page, which will be part of the 2.2 documentation,
provides an introduction and a few guidelines for people that want to
upgrade.

I especially suggest to give a look at the ToscaWidgets2 section as
reading it can avoid many problems like collisions between tw1 and tw2

http://turbogears-dev.readthedocs.org/en/latest/main/TG2Upgrading.html