[llvm] [docs] Expand HowToAddABuilder with guidance on testing locally (PR #115024)

[Alex Bradbury via llvm-commits]

asb wrote:

> This is beside the point of this change but I wonder if this document could do with a note at the start to the effect that the "master" terminology is what the Buildbot project uses and is therefore used here in that context (see [buildbot/buildbot#5382](https://github.com/buildbot/buildbot/issues/5382)).
> 
> Since we have precedent for not using the term ourselves, when we moved the master branch to main.

Yes, I've simply followed what we already do in LLVM by using "buildmaster" as upstream buildbot does (though they did of course rename "buildslave" to "worker"). I'm not sure what the right thing is here.

https://github.com/llvm/llvm-project/pull/115024