[docs] indexing transactions and subscribing to events

Refs #1643

docs/subscribing-to-events-via-websocket.rst

@@ -0,0 +1,28 @@
+Subscribing to events via Websocket
+===================================
+
+Tendermint emits different events, to which you can subscribe via `Websocket
+<https://en.wikipedia.org/wiki/WebSocket>`__. This can be useful for
+third-party applications (for analysys) or inspecting state.
+
+`List of events <https://godoc.org/github.com/tendermint/tendermint/types#pkg-constants>`__
+
+You can subscribe to any of the events above by calling ``subscribe`` RPC method via Websocket.
+
+::
+
+    {
+        "jsonrpc": "2.0",
+        "method": "subscribe",
+        "id": "0",
+        "params": {
+            "query": "tm.event='NewBlock'"
+        }
+    }
+
+Check out `API docs <https://tendermint.github.io/slate/#subscribe>`__ for more
+information on query syntax and other options.
+
+You can also use tags, given you had included them into DeliverTx response, to
+query transaction results. See `Indexing transactions
+<./indexing-transactions.html>`__ for details.