README: add update hints

README.md

@@ -1,6 +1,15 @@
 > ℹ️ Issues for this repository are tracked on [Phabricator](https://phabricator.wikimedia.org/project/board/5563/) - ([Click here to open a new one](https://phabricator.wikimedia.org/maniphest/task/edit/form/1/?tags=wikibase_cloud
 ))
 
+This repo is a fork of https://github.com/magnusmanske/quickstatements
+
+The Dockerfile build process runs composer and pulls a version the wbstack fork of magnus-tools from https://github.com/wbstack/magnustools/
+This is then copied out of `/vendor` by a `composer` post-update/install hook and then finally copied into the correct place in the final image.
+
+To update this repository you need to pull the updates from the upstream and ensure our customisations are working on them. This may be easiest to do with a `git pull --rebase` from upstream. You may also need to update our magnustools fork for compatibility and then use that updated version.
+
+If you go through this process please also update here to document it
+
 Using addwiki:
 https://github.com/addwiki/mediawiki-api