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

[David Spickett via llvm-commits]

DavidSpickett 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 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.

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