Updated README with install and usage info

Updated README with install and usage info. Let me know if I've left any key pieces out.

Unless I'm mistaken, I don't see a way to grab branch info, just the short sha.

If I'm correct, I was thinking about adding that in.

I was also thinking it might be nice to have a numeric version number, something that could easily be called out by a client in addition to the hash.

I was looking at: git rev-list --count HEAD

What do you think?