Instructions to test-drive unreleased code

#42 
I think this is going to lead to problems further down the line (and user support questions). The master version of one gem might be incompatible with the master version of another gem.

Such as recently master CLAide was [incompatible with CocoaPods master](https://github.com/CocoaPods/CocoaPods/issues/2819) (and that was even across CocoaPods 0.35 release). I think we should mention this. Or perhaps provide a better way. Ideally the commits of the other gems locked from [Gemfile.lock](https://github.com/CocoaPods/CocoaPods/blob/master/Gemfile.lock#L1-63) inside CocoaPods master should be used.