Update README and docs.

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.

CONTRIBUTING.md

@@ -0,0 +1,21 @@
+## License
+
+Copyright (c) 2019 Martin Algesten
+
+Licensed under either of
+
+ * Apache License, Version 2.0
+   ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license
+   ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+## Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed under the Apache License, Version 2.0, and the MIT license, without
+any additional terms or conditions. See LICENSE-APACHE and LICENSE-MIT for
+details.
+