As for the first part anyone who would ask you that probably hasn't heard of the Dunning-Kruger effect, and you should respond by educating them (politely) that you providing such a rating probably isn't very useful http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect

The second part can easily be achieved by linking to your github profile. If you don't contribute to any open source projects you probably have some side projects you wouldn't mind making public on github (if not, then start one).