examples: added README.md to ipc_pingpong example

examples/ipc_pingpong/README.md

+IPC Pingpong!
+=============
+
+This example is to illustrate the usage of RIOTs IPC messaging system.
+
+The application starts a second thread (in addition to the main thread) and sends messages between
+these two threads. The main thread call `thread_send_receive()` in an endless loop. The second
+thread receives the message, prints `2nd: got msg from x` to stdout and sends a reply message with
+an incrementing number back to the main thread.
+
+The correct output should look like this:
+```
+kernel_init(): This is RIOT! (Version: xxx)
+kernel_init(): jumping into first task...
+Hello world!
+second_thread starting.
+2nd: got msg from 1
+Got msg with content 2
+2nd: got msg from 1
+Got msg with content 3
+2nd: got msg from 1
+Got msg with content 4
+2nd: got msg from 1
+Got msg with content 5
+2nd: got msg from 1
+Got msg with content 6
+2nd: got msg from 1
+Got msg with content 7
+2nd: got msg from 1
+Got msg with content 8
+2nd: got msg from 1
+Got msg with content 9
+2nd: got msg from 1
+Got msg with content 10
+2nd: got msg from 1
+[...]
+```