Add links to documentation sources.

Linking directly to the relevant/example document source locations makes it easier for potential contributors to find them.

Author: follower

src/contributing.md

@@ -373,10 +373,13 @@ the submodule to. Running `./x.py build` should work now.
 ## Writing Documentation
 
 Documentation improvements are very welcome. The source of `doc.rust-lang.org`
-is located in `src/doc` in the tree, and standard API documentation is generated
-from the source code itself. Documentation pull requests function in the same way
+is located in  [`src/doc`][srcdoc] in the tree, and standard API documentation is generated
+from the source code itself (e.g. [`lib.rs`][librs]). Documentation pull requests function in the same way
 as other pull requests.
 
+[srcdoc]: https://github.com/rust-lang/rust/tree/master/src/doc
+[librs]: https://github.com/rust-lang/rust/blob/master/library/std/src/lib.rs#L1
+
 To find documentation-related issues, sort by the [T-doc label][tdoc].
 
 [tdoc]: https://github.com/rust-lang/rust/issues?q=is%3Aopen%20is%3Aissue%20label%3AT-doc