[mruby] contributing to the mruby clone repo

[Jon]

For anyone contributing to the TCS mruby repo, please place code in
namespaced topic branches using the first letter of your first and
last name rather than commit to `master`. The `master` branch will
always be kept in sync with the official mruby repo.

For example, `jf/foo`, `jf/bar`, `jf/xcode`, `jf/xcode-cross`, `jf/vs11`, etc...

Once we like the code enough to submit a pull request back upstream,
we'll create a new non-namespaced branch (e.g. - `xcode`), do any
required commit sculpting to clean things up, then submit.

Thanks, Jon