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

[David Spickett via llvm-commits]

DavidSpickett wrote:

> I'm not sure what the right thing is here.

Adding a note at the start would be my choice, I'll make a PR for it.

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