Markdown Maintenance status badges

https://doc.rust-lang.org/cargo/reference/manifest.html#the-badges-section

crates.io previously displayed badges next to a crate on its website, but that functionality has been removed. Packages should place badges in its README file which will be displayed on crates.io

rust-lang/crates.io#2438

• actively-developed: New features are being added and bugs are being fixed.

  

  ![maintenance-status](https://img.shields.io/badge/maintenance-actively--developed-brightgreen.svg)

• passively-maintained: There are no plans for new features, but the maintainer intends to respond to issues that get filed.

  

  ![maintenance-status](https://img.shields.io/badge/maintenance-passively--maintained-yellowgreen.svg)

• as-is: The crate is feature complete, the maintainer does not intend to continue working on it or providing support, but it works for the purposes it was designed for.

  

  ![maintenance-status](https://img.shields.io/badge/maintenance-as--is-yellow.svg)

• experimental: The author wants to share it with the community but is not intending to meet anyone's particular use case.

  

  ![maintenance-status](https://img.shields.io/badge/maintenance-experimental-blue.svg)

• looking-for-maintainer: The current maintainer would like to transfer the crate to someone else.

  

  ![maintenance-status](https://img.shields.io/badge/maintenance-looking--for--maintainer-orange.svg)

• deprecated: The maintainer does not recommend using this crate (the description of the crate can describe why, there could be a better solution available or there could be problems with the crate that the author does not want to fix).

  

  ![maintenance-status](https://img.shields.io/badge/maintenance-deprecated-red.svg)