Clarify API level in README

It took me ages to work out why somebody would create a compatibility library in 2013 for a feature added to Android in version 4 until I clicked through to the Android documentation and saw API Level 14. I have added the API level to the README to make this more clear.

I'm so used to people saying ICS that "Android 4" hardly means anything to me!
Merge pull request #3 from intrications/patch-1

Clarify API level in README