Link to non-rustc components

[mark-i-m]

At some point, it would be nice to add documentation for the following. I'm not really sure how to do this, since the rustdocs for these crates are not built or host anywhere, to my knowledge.

Special thanks to @rajcspsg for compiling this list in #100 (comment)

s#	FileName	github Link	line#
1	compiletest.md	https://github.com/rust-lang/rust/tree/master/src/tools/compiletest/src/runtest.rs	186
2	compiletest.md	https://github.com/rust-lang/rust/tree/master/src/tools/compiletest/src/header.rs	234
3	compiletest.md	https://github.com/rust-lang/rust/tree/master/src/tools/compiletest/src/common.rs	235
4	intro.md	https://github.com/rust-lang/rust/tree/master/src/bootstrap/test.rs	169
5	appendix-stupid-stats.md	https://github.com/rust-lang/rust/tree/master/src/librustc_driver/lib.rs	114
6	traits-lowering-module.md	https://github.com/rust-lang/rust/tree/master/src/test/ui/chalkify/lower_impl.rs	64
7	adding.md	https://github.com/rust-lang/rust/tree/master/src/tools/compiletest/src/header.rs	166
8	adding.md	https://github.com/rust-lang/rust/blob/master/src/test/ui/hello_world/main.rs	248
9	adding.md	https://github.com/rust-lang/rust/blob/master/src/test/ui/transmute/main.rs	312

[mark-i-m]

It looks like we still don't have a better solution than to link to these directly, and frankly it seems to work fine, so I will close this.