Add documentation

docs/config/inode.en.md

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+[Documentation](../../README.md#documentation) → [Configuration](../config.en.md) → Image metadata in etcd
+
+-----
+
+[Читать на русском](inode.ru.md)
+
+# Image metadata in etcd
+
+Image list is stored in etcd in `/vitastor/config/inode/<pool>/<inode>` keys.
+
+You can even create images manually:
+
+```
+etcdctl --endpoints=<etcd> put /vitastor/config/inode/<pool>/<inode> '{"name":"<name>","size":<size>[,"parent_id":<parent_inode_number>][,"readonly":true]}'
+```
+
+For example:
+
+```
+etcdctl --endpoints=http://10.115.0.10:2379/v3 put /vitastor/config/inode/1/1 '{"name":"testimg","size":2147483648}'
+```
+
+If you specify parent_id the image becomes a CoW clone. I.e. all writes go to the new inode and reads first check it
+and then upper layers. You can then make parent readonly by updating its entry with `"readonly":true` for safety and
+basically treat it as a snapshot.
+
+So to create a snapshot you basically rename the previous upper layer (for example from testimg to testimg@0), make it readonly
+and create a new top layer with the original name (testimg) and the previous one as a parent.
+
+vitastor-cli, K8s, OpenStack and other drivers also store the reverse mapping in `/vitastor/index/image/<name>` keys
+in JSON format: `{"id":<inode>,"pool_id":<pool>}` and ID counters in `/vitastor/index/maxid/<pool>` as numbers
+to simplify ID generation.