Define how the TensorFlow Quantum project uses labels on GitHub issue & PRs

[mhucka]

Is your feature request related to a use case or problem?

We use issue and PR labels in this project. The descriptions of the labels in GitHub explains what most of them are for, but we should spell out in better detail when, how, and why they are meant to be applied.

What solution or approach do you envision?

This kind of information should go in a contribution/developer guide similar to Cirq's. It may be something that could go in CONTRIBUTING.md, but that file is getting long, so it may be too much to put it there.

How urgent is this for you?

None

[rosspeili]

Could be a compact seperate doc linked via contributing. Have been working on something similar. Happy to pick this if you'd like @mhucka, but I'll need some input on what each label implies, besides the obvious ones. Could create a draft by eod and take it from there.