fix(ds): Update README

apps/emqx_durable_storage/README.md

@@ -1,36 +1,50 @@
 # EMQX Replay
 
-`emqx_ds` is a generic durable storage for MQTT messages within EMQX.
+`emqx_ds` is an application implementing durable storage for MQTT messages within EMQX.
 
-Concepts:
+# Features
 
+- Streams. Stream is an abstraction that encompasses topics, shards, different data layouts, etc.
+  The client application must only aware of the streams.
 
+- Batching. All the API functions are batch-oriented.
 
-> 0. App overview introduction
+- Iterators. Iterators can be stored durably or transferred over network.
-> 1. let people know what your project can do specifically. Is it a base
+  They take relatively small space.
-> library dependency, or what kind of functionality is provided to the user?
-> 2. Provide context and add a link to any reference visitors might be
-> unfamiliar with.
-> 3. Design details, implementation technology architecture, Roadmap, etc.
 
-# [Features] - [Optional]
+- Support for various backends. Almost any DBMS that supports range
-> A List of features your application provided. If the feature is quite simple, just
+  queries can serve as a `emqx_durable_storage` backend.
-> list in the previous section.
+
+- Builtin backend based on RocksDB.
+  - Changing storage layout on the fly: it's achieved by creating a
+    new set of tables (known as "generation") and the schema.
+  - Sharding based on publisher's client ID
 
 # Limitation
-TBD
+
+- Builtin backend currently doesn't replicate data across different sites
+- There is no local cache of messages, which may result in transferring the same data multiple times
 
 # Documentation links
 TBD
 
 # Usage
-TBD
+
+Currently it's only used to implement persistent sessions.
+
+In the future it can serve as a storage for retained messages or as a generic message buffering layer for the bridges.
 
 # Configurations
-TBD
+
+`emqx_durable_storage` doesn't have any configurable parameters.
+Instead, it relies on the upper-level business applications to create
+a correct configuration and pass it to `emqx_ds:open_db(DBName, Config)`
+function according to its needs.
 
 # HTTP APIs
 
+None
+
 # Other
 TBD