Add devel-docs/performance-logs/performance-logs.md, which
describes how to record and view performance logs, and how to
report perofrmance-related issues.
This will be a neat reminder for some of the hidden environment
variables helpful to debug GIMP in some cases, as well as some other
tricks, which we often have to tell people or even which we forget (or
don't even know sometimes) ourselves.
This will be less annoying to just remember to check this file if we
forget the name of some environment variable than to try and grep the
git log or the code.
If someone adds another environment variable which changes some behavior
of the code, please add it here. Also if you know some other helpful
tricks, share with all. Thanks.
