Added import explanation

It took me 2 hours and a lot of searches before finding a sample project showing me the right way of importing..

If the regular import "Reachability.h" does not working, search does not really help and only brings up topics like this one: 

http://stackoverflow.com/questions/12002905/ios-build-fails-with-cocoapods-cannot-find-header-files
Merge pull request #34 from tiemevanveen/master

Better import explanation