Add docs for crate keyword

I think it might be used in some other things, but I'm not fluent enough
at sifting through the rust compiler's source code to find every use of
a specific keyword.

This leaves the question of how to document the `extern` keyword, what
with how much overlap it has with `crate`, but that's used with ABI
stuff so that should be fine.

Author: iirelu

src/libstd/keyword_docs.rs

@@ -90,6 +90,40 @@ mod as_keyword { }
 /// [Reference]: https://doc.rust-lang.org/reference/items/constant-items.html
 mod const_keyword { }
 
+#[doc(keyword = "crate")]
+//
+/// The `crate` keyword.
+///
+/// The primary use of the `crate` keyword is as a part of `extern crate` declarations, which are
+/// used to specify a dependency on a crate external to the one it's declared in. Crates are the
+/// fundamental compilation unit of Rust code, and can be seen as libraries or projects. More can
+/// be read about crates in the [Reference].
+///
+/// ```rust ignore
+/// extern crate rand;
+/// extern crate my_crate as thing;
+/// extern crate std; // implicitly added to the root of every Rust project
+/// ```
+///
+/// The `as` keyword can be used to change what the crate is referred to as in your project. If a
+/// crate name includes a dash, it is implicitly imported with the dashes replaced by underscores.
+///
+/// `crate` is also used as in conjunction with [`pub`] to signify that the item it's attached to
+/// is public only to other members of the same crate it's in.
+///
+/// ```rust
+/// # #[allow(unused_imports)]
+/// pub(crate) use std::io::Error as IoError;
+/// pub(crate) enum CoolMarkerType { }
+/// pub struct PublicThing {
+///     pub(crate) semi_secret_thing: bool,
+/// }
+/// ```
+///
+/// [Reference]: https://doc.rust-lang.org/reference/items/extern-crates.html
+/// [`pub`]: keyword.pub.html
+mod crate_keyword { }
+
 #[doc(keyword = "fn")]
 //
 /// The `fn` keyword.