[Telepathy] Explaining what's changed since the last draft when undrafting interfaces

[Will Thompson]

Hi,

It would be kinder to maintainers of non-core Telepathy components if 
the <tp:added/> for freshly-undrafted interfaces (and also the NEWS 
file, maybe?) included a quick summary of what's changed since the last 
draft. This is easier than rootling through `git log -p` for the 
relevant files.

(I think the slight clutter this would add to the spec would be worth it.)

.o/,
-- 
Will