add note on previewgenerator

README.md

@@ -166,3 +166,25 @@ Here is an example CSS config which hides the local login and makes space for a
 [nextcloud-docker]: https://hub.docker.com/_/nextcloud/
 [`abra`]: https://git.autonomic.zone/autonomic-cooperative/abra
 [`coop-cloud/traefik`]: https://git.autonomic.zone/coop-cloud/traefik
+
+## Using [`previewgenerator`](https://github.com/nextcloud/previewgenerator) app
+
+After you install, enable etc. then you need to run the generation (**warning**: it can take a long time!):
+
+```
+abra app run <domain> app bash -u www-data
+./occ preview:generate-all
+```
+
+To set up the cron to run again, there is [no clear solution in the context of
+containers](https://github.com/nextcloud/previewgenerator/issues/1). So, a
+pretty dodgy hack is to run it from the system directly:
+
+```
+root@foo.com /etc/cron.hourly $ cat foo-com-preview-generate 
+#!/bin/bash
+
+docker exec -u www-data $(docker ps -f name=foo_com__app -q) ./occ preview:pre-generate
+```
+
+This app will improve performance of image browsing at the cost of storage space.