Add how-to document for applying Pantheon updates

** Why are these changes being introduced:

* The process of applying upstream changes made by Pantheon to their
  platform has proven to be somewhat complex through the first three
  rounds of updates. We need a how-to document to help us follow a
  consistent workflow.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/pw-49

** How does this address that need:

* This writes out the steps I've been following, particularly for the
  last two rounds of updates. This is the starting point for this work,
  as I'm not certain that I've found a reasonably simple workflow.

** Document any side effects to this change:

* None - this is a documentation-only change.

docs/howto/apply-pantheon-updates.md

@@ -2,3 +2,49 @@
 
 This document will describe the process for keeping our repository up-to-date
 with upstream changes introduced by Pantheon.
+
+## Background
+
+Our GitHub repository was created as a fork of Pantheon's [WordPress Composer
+Managed](https://github.com/pantheon-systems/wordpress-composer-managed) repository. As Pantheon continues to develop their platform, they
+merge changes into their `default` branch - which we need to then incorporate
+into our `master` branch.
+
+## Workflow
+
+1. Create a "catchup" branch from `master`, and create a multidev from it.
+
+Doing this now, from a trusted starting point, can make troubleshooting easier.
+
+2. Sync our `default` branch - and only the `default` branch - with the Pantheon
+   repository. **Do not sync the entire repository**
+
+3. Identify which commits on the `default` branch still need to be merged
+
+Each round of platform updates is marked along the branch with a tag - so this
+step should be completed by reading the list of commits until you get to a tag.
+
+4. Review this set of commits to the `default` branch to identify any concerns
+
+Ideally all commits will be judged safe to apply. If a subset are desired, pick
+that point along the `default` branch.
+
+5. Create a "default#" branch off of the chosen starting point
+
+The number used in this branch name will naturally change over time, obviously.
+
+6. Merge "default#" into "catchup"
+
+This PR can be merged without review, but may require resolving a merge
+conflict in cases where Pantheon makes changes to a file we have also modified.
+
+7. Re-deploy "catchup" to Pantheon, and inspect it for functionality
+
+8. After any needed signoffs and review, merge "catchup" into `master`
+
+9. Create a tag on the `default` branch marking what was included in this round
+   of updates, to make the next round of updates easier to start.
+
+The name for this tag should match the `default#` branch you created in step 5.
+
+10. Delete the catchup and default# branches you created during this process.