Merge branch 'maintpolicy' into blead

Merge branch 'maintpolicy' into blead

Update our policies w.r.t maintenance releases in the light of discussions
had during the release of 5.20.2, in particular:

http://www.nntp.perl.org/group/perl.perl5.porters/2014/12/msg224162.html
http://www.nntp.perl.org/group/perl.perl5.porters/2014/12/msg224120.html
http://www.nntp.perl.org/group/perl.perl5.porters/2015/01/msg224680.html

The lists of acceptable/unacceptable changes are essentially unchanged, but
the wording now emphasizes that it may not be desirable to cherry-pick
every eligible change, and states which types of change are the most
important ones to focus on.

The intention is to draw up a much shorter list of cherry-pick proposals in
the future. Hopefully, a much less daunting list of proposals will reduce
the difficulty in getting the required number of votes on the ones that
really matter and possibly reduce risk slightly in the process, but not
affect how worthwhile the contents of maintenance releases are.