add profiling instructions

[jamestalmage]

This adds a maintaining.md file where we can store information helpful to maintainers.

The first entry is a set of instructions for self-profiling, based on my comments here.

[bcoe]

@jamestalmage this is great 👍

My only thought, perhaps we can move it into a /docs directory, I bet we'll have more documents like this we add over time.

[bcoe]

@jamestalmage if you don't mind moving this into a /docs folder, and rebasing, let's land this.