[Releasing] [Development] Change file process & improvement proposal

[Edward Welbourne]

> Proposal: Check for [ChangeLog] entries in a 5.x branch _after 5.x.0 branched_. Is that easy to enable/disable?

It should be easy to detect that 5.x.0 "has branched" - check for
existence of either origin/5.x.0 branch or v5.x.0 tag.  That we're
committing to 5.x is fairly easy *in Gerrit* (with its relatively
conservative branch set), and checking for 5.x when folk are using that
locally for development is easy, but checking a topic-branch for whether
it's based on (so presumptively targetting) 5.x rather than 5.y or dev
may be trickier.  (Getting such checks through code review may also be
tricky; c.f. https://codereview.qt-project.org/177706 ...)

	Eddy.