Add note at the top of updating a package asking to check for existing maintainers before starting

[TraceyC77]

We need to add a blub to the top of Updating an Existing Package to check the MAINTAINERS file before getting started

We had a PR for a package maintained by algent, where the submitter hadn't checked with him
Given that this is the beginning of the Google Summer of Code, we're likely to see this happen more in the near future

[liontiger23]

This is fixed with #650 right?

[malfisya]

This is fixed with #650 right?

Yes