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 generator [see forum post](http://devnet.jetbrains.com/thread/451790?tstart=0).

I think this is because 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