docs/images: Document composefs here too

This needs to be called out way more strongly than it is now.
For example a lot of our assumptions about being able to put content anywhere
(i.e. outside of `/usr` which happens especially with containers) which rely on it.

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

docs/src/bootc-images.md

@@ -147,3 +147,18 @@ which few domains can access.
 References:
 
 - <https://github.com/ostreedev/ostree-rs-ext/issues/510>
+
+## composefs
+
+It is strongly recommended to enable the ostree composefs
+backend (but not strictly required) for bootc.
+Do so by ensuring you have this in your `/usr/lib/ostree/prepare-root.conf`:
+
+```
+[composefs]
+enabled = yes
+[sysroot]
+readonly = true
+```
+
+More in [ostree-prepare-root](https://ostreedev.github.io/ostree/man/ostree-prepare-root.html).