Add PR template

Author: aramprice

.github/pull_request_template.md

@@ -0,0 +1,9 @@
+NOTE: this repository uses a "Merge Forward" strategy
+
+Changes should be made in the earliest applicable branch, and
+merged forward through subsequent branches.
+1. Create a PR into the oldest branch (`ubuntu-<short_name_0>`)
+2. After this PR has been merged create a `merge-to-<next-short_name>` branch
+3. Merge `ubuntu-<short_name_0>` into `merge-to-<next-short_name>`
+4. Create a PR to merge `merge-to-<next-short_name>` into `ubuntu-<next-short_name>`
+5. Repeat as needed for subsequent branches