Add notice to users admin panel

[douglas-johnson]

Background

When adding a Guest Author, if a WordPress User with that exact name already exists, you'll get an error and the Guest Author will not be created.

We avoid that by making all usernames lowercase and alloneword. So far we've only asked people to remember to do it that way and name conflicts still happen.

Details

• Add a message to the add user form that describes the necessary formatting.