Changelog for NIFTy_6

Dear @parras, @pfrank, @phaim and @mtr,

should we introduce a changelog for the NIFTy_6 branch?

Currently, there are 107 files changed w.r.t. NIFTy 5, but no indication which of these changes affect the user interface.

If there was a list of (compatibility breaking) changes to the UI, NIFTy 5 users who want to update their code (now or in the distant future) could do so without testing by trial and error which changes they need to make.

Currently, your memory of the changes is probably still fresh enough so you could create such a changelog without excessive trouble, and it would be a very valuable feature to everyone else (and spare you explaining it over and over again).

What do you think? Would you be willing to write up a short summary of your UI changes?
And how about a policy "No merges into NIFTy_6_without a UI changelog entry (if applicable)" for the future?

Thank you all for building the multi frequency capabilities and for considering this,

Lukas