Document that you don't need to build the compiler to work on libraries

[jyn514]

This is probably already there somewhere, but it should be near the very start of the guide.

cc @thomcc

[jyn514]

The 'somewhere' is https://rustc-dev-guide.rust-lang.org/tests/running.html#run-tests-on-the-standard-library. It should be moved to 'How to build and run the compiler' I think.

[jyn514]

It appears to also be documented at https://rustc-dev-guide.rust-lang.org/getting-started.html#building-and-testing-stdcorealloctestproc_macroetc. Neither is its own page.