Add README

[matthewjasper]

See title.

Some of the questions that I would hope it would answer are:

• All of the usual things about building and contributing.
• If and how are larger contributions and rewrites accepted?
• What are the main goals for improving the reference?

[steveklabnik]

Absolutely!

[est31]

Bump. Just explained someone on IRC how to invoke mdbook to build the reference, which wouldn't have been required had there been a README.

[brauliobz]

I didn't put the --git https://github.com/azerupi/mdBook.git when installing mdbook. Is it required?

[steveklabnik]

@brauliobezerra no, it's not; any version of gitbook works, and there's no real reason to prefer the version from git.