Auto merge of #3999 - kureuil:cratesio-676, r=alexcrichton
authorbors <bors@rust-lang.org>
Sun, 7 May 2017 18:03:51 +0000 (18:03 +0000)
committerbors <bors@rust-lang.org>
Sun, 7 May 2017 18:03:51 +0000 (18:03 +0000)
Document new behavior of crates.io with the documentation field of the manifest

[crates.io](https://crates.io/) now automatically links a crate with its [docs.rs](https://docs.rs/) page if no documentation link was specified in the manifest.

See rust-lang/crates.io#676


Trivial merge