Notify existing maintainers when a new maintainer is added

[Segaja]

It would be nice if existing maintainers of a package would be notified (via email), if a new maintainer is being added.

It would help to make the existing maintainers aware that someone new joined the effort of maintaining the package in question.

[jelly]

This should probably become a user setting as not everyone might enjoy getting tons of emails (like @felixonmars)