Add section about self-profiling and measureme

[michaelwoerister]

The in-tree documentation about this is not too bad but it would be good to give a high-level overview here, and especially add a step-by-step guide how to add new measurements (what needs to land in measureme and what in the compiler and how to sequence that).

[jyn514]

There's already a section on measureme here: https://rustc-dev-guide.rust-lang.org/profiling.html

It only has docs on how to use the existing measurements, not to add new ones, but I think that might be better suited to somewhere in https://github.com/rust-lang/measureme/ instead?