Add information on running the compiler with cachegrind and valgrind

[rylev]

As a follow-up to #88, we should include more specifics on tools like cachegrind and valgrind. From the original issue:

those [tools] provide a slightly different and useful set of measurements but come with some caveats when invoking the profiler (i.e. making sure that the rustc executable is profiled, not cargo or the rustup wrapper).

[jyn514]

I expect something like env RUSTC="valgrind $(rustup which rustc)" should work in most cases, or if that syntax doesn't work then a wrapper script that does the same.