update readme with example

README.md

@@ -1 +1,79 @@
-See docs https://godoc.org/github.com/micromdm/mdm
+[![GoDoc](https://godoc.org/github.com/micromdm/mdm?status.svg)](http://godoc.org/github.com/micromdm/mdm) [![Build Status](https://travis-ci.org/micromdm/mdm.svg?branch=master)](https://travis-ci.org/micromdm/mdm)
+
+The mdm package holds structs and helper methods for payloads in Apple's Mobile Device Management protocol.  
+This package embeds the various payloads and responses in two structs - `Payload` and `Response`.
+
+# How an MDM server executes commands on a device.
+To communicate with a device, an MDM server must create a Payload property list with a specific RequestType and additional data for each request type. Let's use the DeviceInformation request as an example:
+
+
+```
+    // create a request
+	request := &CommandRequest{
+		RequestType: "DeviceInformation",
+		Queries:     []string{"IsCloudBackupEnabled", "BatteryLevel"},
+	}
+
+    // NewPayload will create a proper Payload based on the CommandRequest struct
+	payload, err := NewPayload(request)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Encode in a plist and print to stdout
+    // uses the github.com/groob/plist package
+	encoder := plist.NewEncoder(os.Stdout)
+	encoder.Indent("  ")
+	if err := encoder.Encode(payload); err != nil {
+		log.Fatal(err)
+	}
+```
+
+Resulting command payload:
+```
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+  <dict>
+    <key>Command</key>
+    <dict>
+      <key>Queries</key>
+      <array>
+        <string>IsCloudBackupEnabled</string>
+        <string>BatteryLevel</string>
+      </array>
+      <key>RequestType</key>
+      <string>DeviceInformation</string>
+    </dict>
+    <key>CommandUUID</key>
+    <string>fa34b4b7-0553-4b3a-9c4b-76b8b357a622</string>
+  </dict>
+</plist>
+```
+
+An MDM server will queue this request and send a push notification to a device. When device checks in, the server will
+reply with the queued plist.
+
+Once the device receives and processes the payload plist, it will reply back to the server. The response will be another plist, which can be unmarshalled into the `Response` struct. Below is the response to our DeviceInformation request.
+
+```
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>CommandUUID</key>
+    <string>fa34b4b7-0553-4b3a-9c4b-76b8b357a622</string>
+	<key>QueryResponses</key>
+	<dict>
+		<key>BatteryLevel</key>
+		<real>1</real>
+		<key>IsCloudBackupEnabled</key>
+		<false/>
+	</dict>
+	<key>Status</key>
+	<string>Acknowledged</string>
+	<key>UDID</key>
+	<string>1111111111111111111111111111111111111111</string>
+</dict>
+</plist>
+```