Update README.md

Author: l4rm4nd

examples/transfer.sh/README.md

@@ -5,3 +5,5 @@
 # Notes
 
 Ensure that the bind mount volume can be written by the container.
+
+The provided container image uses `5000` as PUID and PGID as default. If you want to change those, you have to build the container image locally. Read [here](https://github.com/dutchcoders/transfer.sh#building-the-container) for more details.