Improve README by generating from rustdoc
Currently, the README for each crate is very modest, and only mentions the reference to docs.rs and the license. (mwbot doesn't even exist!)
However, when looking for a crate, crates.io is commonly browsed, and we think it is very reasonable to write "what this crate can do" in the README. This would be approximately in line with the crate-level rustdoc.
In this MR, we have configured cargo-readme to automatically update the README at release time.
If you don't like this approach, don't worry about it and close it.