Auto merge of #4968 - edunham:patch-1, r=alexcrichton

Link to crate type docs

This page (served at https://doc.rust-lang.org/cargo/reference/manifest.html) is the first hit I get when searching "cargo crate-type". The doc which actually explains `crate-type`, https://doc.rust-lang.org/reference/linkage.html, does not appear in the first page of results. I hope that linking the reference doc here will make it easier for others to find instead of having to dig into closed GitHub issues like I did to find it.