add s3 -> local storage migration guide

README.md

@@ -63,6 +63,25 @@ revisions to (instead of deleting them).
 
 _As of 2022-03-30, this requires `abra` RC version, run `abra upgrade --rc`._
 
+### Migrate from S3 to local storage
+
+- `abra app config <domain>`, add
+    COMPOSE_FILE="$COMPOSE_FILE:compose.local.yml"
+    FILE_STORAGE_UPLOAD_MAX_SIZE=26214400
+- `abra app deploy <domain> -f`
+  - compose.aws.yml should still be deployed!
+- `abra app undeploy <domain>`
+- on the docker host, find mountpoint of newly created volume via `docker volume ls` and `docker volume inspect`
+  - volume name is smth like `<domain>_storage-data`
+- take note which linux user owns `<storage_mountpoint>` (likely `1001`)
+- use s3cmd/rclone/... to sync your bucket to `<storage_mountpoint>`
+- `chown -R <storage_user>:<storage_user> <storage_mountpoint>`
+- `abra app config <domain>`, switch storage backend
+  - remove `AWS_*` vars, `SECRET_AWS_SECRET_KEY_VERSION` and `COMPOSE_FILE="$COMPOSE_FILE:compose.aws.yml"`
+  - set `FILE_STORAGE=local`
+- `abra app deploy <domain> -f`
+- enjoy getting rid of S3 🥳
+
 ## Single Sign On with Keycloak
 
 `abra app config YOURAPPNAME`, then uncomment everything in the `OIDC_` section.