Document backwards-incompatible changes.

[martey]

Commit 11f697d13757be5505898ae2f5444c394ab3b5ae included a number of changes that are backwards-incompatible with previous versions of symposion, including changing template locations. This makes upgrading difficult.

I think it would be good if symposion had a CHANGELOG that included all major changes, but especially any backwards-incompatible ones. If this is acceptable to the project maintainers, I am willing to create a pull request with one.

[jtauber]

Would LOVE your help Martey! We're planning on adopting a distribution and versioning scheme described in https://github.com/pinax/pinax/issues/84 and I think that will help us a lot. A CHANGELOG and migration guide will be a key part of releases from that point on.

We'll need help getting the symposion apps and dependencies ready for the 16.4 release.

Please join us on Slack <slack.pinaxproject.com> if you can to discuss further.