(maint) Clarify the merge-up process in COMMITTERS

Without this patch some of the committers are using a process that
involves merging a topic branch based on stable into both `stable` and
`master`.  This is a problem because it's confusing and inconsistent
from other committers who are merging a topic branch into `stable`, then
merging `stable` into `master.

The two processes, while equivalent in nature, produce different and
inconsistent log messages when reviewing `git lg`.  For example, the
following log is produced by the process that does not merge up:

    *   e130aa5 Merge branch 'maint/stable/filebucket_clarifications' 30 minutes ago Hailee Kenney  (HEAD, origin/master, master)
    |\
    | * b301ada Maint: Improve documentation of filebucket attributes 33 minutes ago Nick Fagerlund
    * |   9aa9d69 Merge pull request #1636 from adrienthebo/maint-readme_developer_explain_side_effects_in_tests 3 days ago Hailee Kenney
    |\ \
    * \ \   515cd97 Merge pull request #1639 from Sharpie/18895-fix-runit-documentation 3 days ago Hailee Kenney
    |\ \ \
    | * | | 713bcbe (#18895) Fix documentation in runit provider 3 days ago Charlie Sharpsteen
    |/ / /
    * | |   4c2655f Merge pull request #1637 from jeffweiss/tickets/bug/18393_diffargs_broken_on_aix53 4 days ago Chris Price
    |\ \ \
    | * | | bb86ff6 (#18393) AIX 5.3 doesn't support diff -u 4 days ago Jeff Weiss
    * | | |   325b8bf Merge pull request #1635 from hlindberg/issues/fix-20581 5 days ago Adrien Thebo
    |\ \ \ \
    | |/ / /
    |/| | |
    | | * | a3941d4 (maint) Add documentation on side effects in tests 6 days ago Adrien Thebo
    | * | | 23005e6 (#20581) Correct parser future's validation of query expression 7 days ago Henrik Lindberg
    |/ / /
    | | * edfabfd (packaging) Update PUPPETVERSION to 3.2.0-rc2 7 days ago Matthaus Owens  (3.2.0-rc2)
    * | |   8c0e1b3 Merge branch 'stable' 7 days ago Jeff McCune
    |\ \ \
    | | |/
    | |/|
    | * |   761de6b Merge branch 'fix_activerecord_centos' into stable 7 days ago Jeff McCune

Compare this to the log of the stable branch and observe how there are two very
similar merge commits with the log `Merge branch
'maint/stable/filebucket_clarifications'`  Please observe these similar commit
messages have different commit ID's, e130aa5 and 47fd6f9 respectively, and both
merge commits are bringing in the change contained in b301ada.

    *   47fd6f9 Merge branch 'maint/stable/filebucket_clarifications' into stable 33 minutes ago Hailee Kenney  (origin/stable, stable)
    |\
    | * b301ada Maint: Improve documentation of filebucket attributes 34 minutes ago Nick Fagerlund
    |/
    * edfabfd (packaging) Update PUPPETVERSION to 3.2.0-rc2 7 days ago Matthaus Owens  (3.2.0-rc2)

Using the merge-up process made clear in this patch, the history will look as
follows:

    *   1f79310 Merge branch 'stable' 5 seconds ago Jeff McCune  (HEAD, master)
    |\
    | *   c6f6ffb Merge branch 'fix/stable/clarify_merge_up' into stable 45 seconds ago Jeff McCune  (stable)
    | |\
    | | * 44b999c (maint) Clarify the merge-up process in COMMITTERS 53 seconds ago Jeff McCune  (fix/stable/clarify_merge_up)
    | |/
    * |   e130aa5 Merge branch 'maint/stable/filebucket_clarifications' 35 minutes ago Hailee Kenney  (origin/master)
    |\ \
    | | *   47fd6f9 Merge branch 'maint/stable/filebucket_clarifications' into stable 36 minutes ago Hailee Kenney  (origin/stable)

This minor difference also has the effect of making `git branch --contains
c6f6ffb` much more useful, as both `master` and `stable` will contain merge
commits made into `stable`.