Merge pull request #718 from Altinity/add_state_storage_documentation

Added state storage documentation

Author: subkanthi

doc/img/state_storage.jpg

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+## State Storage
+
+Sink connector is designed to store data about the replication state.
+Currently the only supported state storage is by persisting the information in ClickHouse tables.
+
+### ClickHouse State Storage
+
+To use ClickHouse as a state storage, you need to specify the following configuration properties:
+
+state storage link to img/state_storage.png
+
+
+
+
+
+# Offsets table.(PostgreSQL)
+| Column Name | Description                                                          |
+|-------------|----------------------------------------------------------------------|
+| id          | UUID                                                                 |
+| offset_key  | [\"debezium-embedded-postgres\",{\"server\":\"embeddedconnector\"}]" |
+| offset_val  |                                     |
+| record_insert_seq  |                                                                      |
+| record_insert_ts  |                                                                      |
+
+### offsets_value
+- **lsn_proc** - Last processed LSN
+- **lsn_commit** - Last committed LSN
+- **messageType** - Type of message(INSERT, UPDATE, DELETE)
+-