doc: Added instructions in CONTRIBUTE.MD for adding an upstream to forked repo #17152

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I have 7 commits in the pull request, so I feel like I've done something wrong. Can somebody please advise me if this is normal?

I think you can get rid of merge commits like so (untested):

git merge upstream/master
git reset --soft upstream/master
git commit -m "the commit message of all commits squashed..."

I've added more commits here. I can't seem to get these down. Am I suppose to be rebasing to squash the commits into one?

Yes, the only commit that should be here is your change.