Updated the version to 2.2 and added the changelog.txt.

This is the last step to the SFML 2.2 codebase, lets get to it! :smile: 
It probably should be plural but we'd have to drop the "a", I mean we added more than one function.

It's not required, but it doesn't hurt either. When I look at a changelog, I'm usually interested in finding out what were the most important changes. Having to search for the announcement somewhere else or having to dig through the whole list is rather cumbersome. It might not always be needed but given that the changelog for 2.2 is 130 lines long, I think it's justified.

@TankOs: You do realize that things are split into Bugfixes and Features section already, right?