docs: update v15 migration guide

docs/migrating_to_15.md

@@ -1,5 +1,21 @@
-# Migrating to Bento v14
+# Migrating to Bento v15
 
-TODO
+Bento v15 migrates to Phenopackets v2, and as such requires that Phenopackets are updated and re-ingested.
 
-* Clear WES DB
+Before updating, perform the following steps:
+
+* Shut down Bento with `./bentoctl.bash stop`
+* Remove the Katsu data volume
+* Remove the WES data volume
+
+Then, update your instance:
+
+```bash
+./bentoctl.bash pull
+```
+
+Finally, restart Bento:
+
+```bash
+./bentoctl.bash start
+```