Link to docs.rs in readme

[Luro02]

Most of the docs are hosted on docs.rs, which I can access very fast, but with https://doc.servo.org/tendril/index.html I have to wait quite some time for a page to load.

It could imagine to keep the API documentation link in the readme and add a badge, that links to https://docs.rs/tendril/

An example of such a badge can be seen here https://github.com/KokaKiwi/rust-hex/blob/master/README.md

[brightly-salty]

I think this should be closed? I see this implemented on my end. Probably just forgotten about.