Add maintenance badge docs
Pending merging of [this PR for crates.io](https://github.com/rust-lang/crates.io/pull/952). This should close off [#704 on crates.io](https://github.com/rust-lang/crates.io/issues/704). I've also updated the badge metadata docs and reordered the fields into groups by build, code coverage, and maintenance.
So I've decided to put the list of possible badge options in the documentation, we'll have to find another place to put more detailed descriptions of the available maintenance badges.