add origin/upstream remotes, minor edits

[ghost]

trivial

(Did the most important changes, the document needs still some revisions)

[clay-aptos]

Thank you for these improvements, @abebeos. I am adding our eng leads to confirm we want to encourage forking the repo. As you fixed, we already recommend it later in the README.

[clay-aptos]

confirm we want to encourage forking the repo.

Note that this is standard-procedure for github.

And you even refer to upstream later in the doc

[davidiw](https://github.com/aptos-labs/aptos-core/blob/main/CONTRIBUTING.md#responding-to-reviewer-feedback)

git fetch upstream
git checkout topic
git rebase -i upstream/main

Agreed. Approving. Thanks again for your help!

[clay-aptos]

Noting I still want @davidiw or @gregnazario to confirm still.

[clay-aptos]

Also, @abebeos , we are working on a section to better explain how to work together here:
#6417

I would appreciate your feedback on that. Can name drop you there too. :-)

[clay-aptos]

Far to big, this overloads me immediately. I'll still give it a try tomorrow.

No worries. It's live now:
https://aptos.dev/community/help-index

[davidiw]

eh... it isn't too this flow has always been the bane of my existence...