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

[David Spickett via llvm-commits]

================
@@ -179,6 +179,74 @@ Here are the steps you can follow to do so:
    buildbot.tac file to change the port number from 9994 to 9990 and start it
    again.
 
+Testing a Builder Config Locally
+================================
+
+It's possible to test a builder running against a local version of LLVM's
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DavidSpickett wrote:

The advice I've been given for this sort of writing is to remove all contractions. `'` is fine for possessives but that's about it, pun intended :)

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