Have a CONTRIBUTING.md or general guidelines to be read for contributions

[badosu]

What it says on the tin.

I already don't review PRs often in BAR, so it's good that at least this knowledge is available somewhere. And this is not only about me but to have this knowledge available somewhere and formalize technical guidelines for contribution.

An example of a guideline is to avoid including hardcoded modifiers whenever applicable and using subactions. See examples of this particular guideline here and here.