This makes src/lib.rs the primary source for crate-level documentation.
I've generated README.md with `cargo readme > README.md`. Since links to
specific documentation items should be relative when possible, but must
be absolute in README.md, I've used the new syntax for intra-rustdoc
links
(https://github.com/rust-lang/rfcs/blob/master/text/1946-intra-rustdoc-links.md),
along with a README.tpl that sets up those links to point at the
absolute versions. `cargo readme` uses the README.tpl by default.
I've also rewritten the crate level docs, removing some TODO information
at the bottom, and moving the license information to CONTRIBUTING.md.
