Auto merge of #5031 - matthiaskrgr:readme_docs, r=alexcrichton

readme: add link to the cargo documentation on docs.rs

[Rendered](https://github.com/matthiaskrgr/cargo/blob/readme_docs/README.md)

Background: I was searching for cargo source code doc a while back, found the cargo book and crates.io doc relatively quickly but not the actual source code doc which I only found (after way to much time had passed)  when I looked up the cargo crate on crates.io and found the "Documentation" link :/

Hope this improves the situation a bit in the future.