magit.texi: add beginnings of a Magit info manual

* Restore the relevant bits in the Makefile

`make install` still depended on `docs` which depended on the existence of `magit.info`, so that was broken.  Since it was suggested that documentation is on the 2.1.0 roadmap, instead of fixing the Makefile, I figured it wouldn't hurt to put together a stub for the new manual (it is going to be info, right?).

I put your name on it, beyond that I don't know what license you intend to use, and I just plugged some useful information from README.md into an Overview section.  I didn't look at the old manual as the commit where you removed it mentioned licensing.