Update GitHub doc to mention that we accepts user branches for Stacked PRs

[joker-eph]

This isn't yet a guide on how to do stacked PRs.

[tstellar]

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.

[vitalybuka]

I assume we going to have SPR setup somewhere.

Looks like everyone I asked to try SPR failed to realize
that branch destination is main repo and branchPrefix must be specific.

[spr]
	githubRemoteName = origin
	githubRepository = llvm/llvm-project
	githubMasterBranch = main
	branchPrefix = users/vitalybuka/spr/
	requireTestPlan = false

[joker-eph]

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?

[joker-eph]

@tstellar wanna double check?

[tstellar]

Looks good, thanks.