(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`.