Add migration docs

[petermueller]

To help people fix their data from the bug fixed in #1

It may require some task generators or something that walk them through how to move their files that have been affected

[petermueller]

@achempion and @ulissesalmeida I'd love a chance for us to get on a call together or something depending on time zones and things.

I think this may also require a forum post if we keep it at 0.3 so that people don't upgrade without looking at the changelog, which will need to contain a bunch of breaking change warnings

[achempion]

I believe the new fix breaks the delete action :) As the put action receives the resolved filename from waffle, when delete and url actions require resolving the filename similar to S3 and local adapters with Versioning.resolve_file_name/3.

[ulissesalmeida]

I think the forum post idea is excellent. I'm sorry about the new fix bug, in my job we never delete, so I didn't manually tested it. So it seems our current integration tests doesn't check if things are properly deleted.

If you want to chat, I'm on BST timezone.