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 Homebrew/homebrew#48879.

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