Based on some user feedback it would be really nice for our notifications emails to include the following:

1. the reason why the notification was sent to the user
2. a link to docs about notifications, including how to turn them off

E.g. for the first, the reason might be because they are the package owner in a tag, because the are the build owner, or because they have an explicit notification subscription.

For the second, we likely need to add a doc to link to.