[Doc] Add git rebase tips to the 'Contributing' page in the developer docs

[asfimport]

A recent discussion on the listserv (link below) asked about how contributors should handle rebasing. It would be helpful if the tips made it into the developer documentation somehow. I suggest in the "Contributing to Apache Arrow" page—currently a wiki, but hopefully eventually part of the Sphinx docs ARROW-4427.

Here is the relevant thread:

https://lists.apache.org/thread.html/c74d8027184550b8d9041e3f2414b517ffb76ccbc1d5aa4563d364b6@%3Cdev.arrow.apache.org%3E

Reporter: Tanya Schlusser / @tanyaschlusser
Assignee: Suvayu Ali / @suvayu

Related issues:

• [Doc] Move Confluence Wiki pages to the Sphinx docs (is blocked by)

PRs and other links:

• GitHub Pull Request #7503

_{Note: This issue was originally created as ARROW-4429. Please see the migration documentation for further details.}