(maint) Update COMMITTERS with feature flags and always working responsibilities

Without this patch Andy's reiteration of PR-1604 is not included in the
COMMITTERS document.  This reiteration is:

    I just want to reiterate that we need to work to make sure that
    master and stable are both clean and working at all times. Use of
    feature flags and other mechanisms is how we achieve this. This
    doesn't mean that dead code is allowed: everything needs to be
    usable in some manner at all points in time. Stable is not an
    indication of the build status of that branch. Stable is an
    indication that it will not be receiving new functionality. We need
    to strive to make both master and stable branches green at all
    times. There is never a case in which we "want" or are "ok with"
    master to be failing tests.

This patch adds these comments to the beginning of the COMMITTERS guide.

[ci skip]