add info about config change to changelog snippet

changelog/unreleased/thumbnails-env-doc.md

@@ -2,4 +2,16 @@ Enhancement: Add description tags to the thumbnails config structs
 
 Added description tags to the config structs in the thumbnails service so they will be included in the config documentation.
 
+**Important**
+If you ran `ocis init` with the `v2.0.0-alpha*` version then you have to manually add the `transfer_secret` to the ocis.yaml.
+
+Just open the `ocis.yaml` config file and look for the thumbnails section.
+Then add a random `transfer_secret` so that it looks like this:
+
+```yaml
+thumbnails:
+  thumbnail:
+    transfer_secret: <put random value here>
+```
+
 https://github.com/owncloud/ocis/pull/3752