doc: stable on-disk format

Fix part of rkt#1132

Documentation/devel/on-disk-format.md

@@ -0,0 +1,31 @@
+On disk format
+==============
+
+The data directory is normally `/var/lib/rkt` but rkt can be requested to use
+another directory via the `--dir` command line argument.
+
+#### CAS database
+
+The CAS database is stored in `/var/lib/rkt/cas/db`. The database schema can be
+migrated to newer versions (#706).
+
+#### CAS
+
+The CAS also uses other directories in `/var/lib/rkt/cas/`. To ensure stability
+for the CAS, we need to make sure we don't remove any of those directories or
+make any destructive changes to them. In future versions of rkt, we will make
+sure to be able to read old versions of the CAS.
+
+#### Pods
+
+The pods are stored in `/var/lib/rkt/pods/` as explained in
+[Life-cycle of a pod](https://github.com/coreos/rkt/blob/master/Documentation/devel/pod-lifecycle.md)
+
+Stability of prepared and exited containers is desirable but not as critical as
+the CAS.
+
+#### Configuration
+
+The [configuration](https://github.com/coreos/rkt/blob/master/Documentation/configuration.md)
+on-disk format is documented separately.
+