[docs] Document the staging buildbot

[dylanmckay]

Summary

This also adds docs to suggest that maintainers of buildbots for
experimental backends should use this buildmaster.

[grosser]

This looks good to me.

docs/HowToAddABuilder.rst

 ===================================================================
 How To Add Your Build Configuration To LLVM Buildbot Infrastructure
 ===================================================================
 
 Introduction
 ============
 
 This document contains information about adding a build configuration and
-buildslave to private slave builder to LLVM Buildbot Infrastructure
-`<http://lab.llvm.org:8011>`_.
+buildslave to private slave builder to LLVM Buildbot Infrastructure.
 
+Buildmasters
+============
+
+There are two buildmasters running.
+
+* The main buildmaster at `<http://lab.llvm.org:8011>`_. All builders attached
+  to this machine will notify commit authors every time they break the build.
+* The staging buildbot at `<http://lab.llvm.org:8014>`_. All builders attached
+  to this machine will be completely silent by default when the build is broken.
+  Builders for experimental backends should generally be attached to this
+  buildmaster.
 
 Steps To Add Builder To LLVM Buildbot
 =====================================
 Volunteers can provide their build machines to work as build slaves to
 public LLVM Buildbot.
 
 Here are the steps you can follow to do so:
 
#. Make sure you can actually start the buildslave successfully. Then set
    buildbot documentation for help.  You may want to restart your computer
    to see if it works.
 
 #. Send a patch which adds your build slave and your builder to zorg.
 
    * slaves are added to ``buildbot/osuosl/master/config/slaves.py``
    * builders are added to ``buildbot/osuosl/master/config/builders.py``
 
+   It is possible to whitelist email addresses to unconditionally receive notifications
+   on build failure; for this you'll need to add an ``InformativeMailNotifier`` to
+   ``buildbot/osuosl/master/config/status.py``. This is particularly useful for the
+   staging buildmaster which is silent otherwise.
+
 #. Send the buildslave access name and the access password directly to
    `Galina Kistanova <mailto:[e-mail]>`_, and wait till she
    will let you know that your changes are applied and buildmaster is
    reconfigured.
 
 #. Check the status of your buildslave on the `Waterfall Display
    <http://lab.llvm.org:8011/waterfall>`_ to make sure it is connected, and
    ``http://lab.llvm.org:8011/buildslaves/<your-buildslave-name>`` to see
    if administrator contact and slave information are correct.
 
 #. Wait for the first build to succeed and enjoy.