Mailing lists

For a while we were already using a tiled-commit mailing list to send around changes made to the code. Since today, there is also a tiled-user mailing list for the users.

We made this move because of the increasing number of people using Tiled. Questions and discussions on the list are archived and users will be able to share information or learn about each other's problems. It would also be a place where we can ask users feedback on new things we plan to add to Tiled, and to do announcements about new versions of Tiled.