Upgrade notes for external plugin authors

There've been various changes to beets over time. We should notify the plugin authors of changes they should make in their program to stay compatible with beets as it moves towards python 3.

Some of the changes they need to make are listed on the [Python 3](https://github.com/beetbox/beets/wiki/Python3) wiki page (that list is not yet complete).

Others are:
- dealing with the fallout from the introduction unicode_literals and its removal
- changing `if name == b'name':` to `if name == 'name':`
- many others (we need to list some of the common ones)
- letting us know if their dependencies won't work with python 3
- more (to list later)

Once we collect more information, we can send them a concrete list of changes.