Feature request: attach binaries to github releases

[fitzgen]

First off: thanks for a lovely tool; I use it for a ton of my crates :)

I also include #[test]s to make sure that the readme is up-to-date, and run them in CI, for when folks send me PRs.

I install cargo readme via cargo install cargo-readme in CI. This makes my builds a bit slower than I'd like, since building cargo readme is not super fast. It would be super awesome if I could instead curl the binary from a github release.

We do a similar thing with appveyor and travis for the wasm-pack project, and it might be helpful to look at if/when adding the config for this repo.

https://github.com/rustwasm/wasm-pack/blob/6d376483d7431b3f2db4de82a52b1cbdca872fea/.appveyor.yml#L21-L44

https://github.com/rustwasm/wasm-pack/blob/2c2627d3202335f886f4b66d9b5356449cac1ac3/.travis.yml#L78-L95