WIP how-to document

docs/howto/apply-pantheon-updates.md

@@ -2,3 +2,34 @@
 
 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. Sync our `default` branch with the changes from Pantheon
+
+2. 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
+should involve looking back over the commits to the last tag.
+
+3. Review the subsequent commits to the `default` branch to identify concerns
+
+4. Create a "catchup" branch off of `master`, and create a multidev
+
+5. Create a "default#" branch off of the chosen starting point
+
+6. Merge "default#" into "catchup"
+
+7. Deploy "catchup" to Pantheon, and inspect it for functionality
+
+8. After any needed signoffs and review, merge "catchup" into `master`
+
+9. Tag the starting point for this round to `default`, for use as the next
+   starting point