Add Additional JSON-RPC API document

Additional-JSON-RPC-API.md

@@ -0,0 +1,45 @@
+# Whisper
+
+## MailServer
+
+MailServer is a Whisper node that has a capability to store messages and send them back after receiving a request. The messages are sent using p2p connection.
+
+### `shh_requestMessages`
+
+Returns cached, possibly expired, messages from a MailServer node.
+
+**Parameters**
+1. `Object`: Options describing the request details:
+* `mailServerPeer` - `String`: enode of a MailServer node (must be first added as a peer and allowed to send p2p messages using [`shh_markTrustedPeer`](https://github.com/ethereum/go-ethereum/wiki/Whisper-v5-RPC-API#shh_marktrustedpeer)),
+* `from` - `Number`: unix time from which messages should be collected (inclusive),
+* `to` - `Number`: unix time to which messages should be collected (inclusive),
+* `topic` - `String`: a topic as a string to filter messages,
+* `symKeyID` - `String`: ID of a symmetric key to authenticate to the MailServer.
+
+**Returns**
+
+`Boolean` on success and an error on failure.
+
+**Example**
+```
+// Request
+curl -X POST --data '{
+  "jsonrpc": "2.0",
+  "method": "shh_requestMessages",
+  "params": [{
+    "mailServerPeer": "enode://d25474361659861e9e651bc728a17e807a3359ca0d344afd544ed0f11a31faecaf4d74b55db53c6670fd624f08d5c79adfc8da5dd4a11b9213db49a3b750845e@52.178.209.125:30379",
+    "topic": "0x11223344",
+    "symKeyID": "b874f3bbaf031214a567485b703a025cec27d26b2c4457d6b139e56ad8734cea",
+    "from": 1513939909,
+    "to": 1513939034
+  }],
+  "id": 1
+}'
+
+// Result
+{
+  "id": 1,
+  "jsonrpc": "2.0",
+  "result": true
+}
+```