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: 
"function" (singular)

But do we need highlights? The choice of features is somewhat arbitrary... For iOS and Android we could discuss it, but they are also mentioned at the beginning of the next paragraph. Do we want to mention things twice? We can still emphasize important changes when we announce the version on the forum, but in this technical list of changes it's not required in my opinion.