[llvm] Update GitHub doc to mention that we accepts user branches for Stacked PRs (PR #73774)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

> I assume we going to have SPR setup somewhere.

Yes, that would be a separate doc in my mind.

> Is there a local config we can use to tell git not to pull these branches? If so, it might be good to document that.

This is the "refspec": https://git-scm.com/book/en/v2/Git-Internals-The-Refspec

I spent quite some time on it today, but I can't find a command to specify it...  You can add one during clone, but `	fetch = +refs/heads/*:refs/remotes/origin/*` will always come by default and needs to be removed and replaced by `	fetch = +refs/heads/main:refs/remotes/origin/main` 

I added a `sed` in the getting started, WDYT?

https://github.com/llvm/llvm-project/pull/73774