add Readme to kotlin-stdlib-gen explaining how to run code generation

As an outside contributor I originally had trouble figuring out how to run the code generate [see forum post](http://devnet.jetbrains.com/thread/451790?tstart=0).

I think the reason is the ReadMe file that explains how to do so is located in libraries/stdlib, not in libraries/tools/kotlin-stdlib-gen

So I added a modified version of the ReadMe to kotlin-stdlib-gen