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.