Document running perf against the database

[d]

There are a few scenarios where we found it really valuable to run the linux perf tools to profile the databases:

1. When we recently found out that mirrors go nuts in the middle of installcheck-good, and we want to be good citizens and collect perf data for the storage team
2. When we are thinking about hot spots and bottlenecks in ORCA, and want a scientific way to profile it
3. In the context of some C++ refactoring, we want to look into using perf probe to accurately count the number of invocations of certain functions that we aim to hide away using resource wrappers, but were afraid of introducing performance regressions (the plan B being using a global volatile int64_t to just count ...)

This is not impossible but might be painful without either #41 or #37