Added polkadot storage page into docs

docs/index.rst

@@ -9,6 +9,7 @@ Haskell Web3 Documentation
    getting_started
    ethereum_node_api
    polkadot_node_api
+   polkadot_storage
    ethereum_accounts
    smart_contracts
    ipfs_client_api

docs/polkadot_storage.rst

@@ -0,0 +1,26 @@
+Polkadot Storage
+================
+
+Blockchains that are built with Substrate expose a remote procedure call (RPC) server
+that can be used to query runtime storage. In Haskell Web3 the standard Web3 provider could be used.
+
+Lets try to query Polkadot storage with ``runWeb3'`` function using ``ghci``.
+
+.. code-block:: haskell
+
+   > import Network.Web3.Provider
+   > import Network.Polkadot
+   > runWeb3' (WsProvider "127.0.0.1" 9944) (query "timestamp" "now" [] :: Web3 (Either String Moment))
+   Right (Right 1610689972001)
+
+The ``query`` function arguments is **section** (or module), **method** and list of arguments (for maps and double maps).
+
+.. code-block:: haskell
+
+   query :: (JsonRpc m, Decode a) => Text -> Text -> [Argument] -> m a
+
+Where ``a`` type should be SCALE decodable.
+
+.. note::
+
+    More usage details available in `Polkadot example <https://github.com/airalab/hs-web3/tree/master/examples/polkadot>`_  app.