A small readme is better than nothing.

I think having a readme is a good idea, because nowhere is it said what CBash actually is, but I'm at a bit of a loss ATM what to put in it. I think useful things will come to mind over time, though.

This is a WIP pull request - I've opened it if there's anything you can suggest, and for a bit of discussion. A couple of ideas for content I have are:

* Links to UESP.net and FOPDoc for record definitions.
* Something about the documentation (and lack of...).
* Maybe what's complete and what's not (can link to wiki pages)?