[llvm-mca] add example workflow for source code

[llvm-mca] add example workflow for source code

This is copied from Andrea's text in PR36875:
https://bugs.llvm.org/show_bug.cgi?id=36875

As noted there, this is a hack...but it's a good one!
It's important to show potential workflows up-front
with examples, so customers can copy and experiment
with them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329726 91177308-0d34-0410-b5e6-96231b3b80d8