Add python verion in readme

[a0x]

I saw you already upgraded build.py to support Python 3, that's great.

However, when I run it under Python 3, I found an error that told me Python 3 was not working here, because depot_tools requires Python 2.7 indeed.

So I think it would be great if you add that info in readme, noticing people that only Python 2 works in this repo.