CONTRIBUTING: how to check for existing PRs.
CONTRIBUTING: how to check for existing PRs.

To avoid extra work on the part of contributors and maintainers, this
adds a step to the explanation of how to contribute a fix to a formula,
showing how to check if someone else has already done so, even if it
has not yet finished being tested, reviewed, and released.

Closes #48879.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

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