Auto merge of #3018 - Jake-Shadle:master, r=steveklabnik

docs(manifest): Update crate-types with `cdylib`

Rust 1.11 now supports the `cdylib` crate-type, so added it to the list of options.

Also added a link to the [Linkage](https://doc.rust-lang.org/reference.html#linkage) section in the Rust Reference manual which explains what the different crate types actually mean in practice....though right now it actually doesn't explain what a `cdylib` is, specifically. ;)