add README

README.md

+# Simple ROS-NNG-Test
+
+This project tests the combination of [NNG](https://nng.nanomsg.org/) and ROS.
+
+NNG supports several communication patterns. Currently, *pub-sub* is supported; others may follow.
+
+## Publish-Subscribe
+
+This demonstrator contains two nodes, a publisher and a subscriber.
+The publisher listens to ROS messages.
+Each received message is transmitted to the subscriber using NNG.
+For each received message, the subscriber publishes another ROS message.
+Currently, the transmission works using TCP with the fixed NNG URL `"tcp://localhost:6576"`.
+
+### Running the Example
+- Checkout the project using git.
+  Since the CMake build uses git submodules, do not remove the git information!
+  It is not required to checkout the submodule manually, this is done using CMake. 
+- Build the project.
+- Start ROS
+  ```
+  $ roscore
+  ```
+- Start the `publisher` and `subscriber` nodes in two terminals.
+  Both are in the `nng_test` namespace.
+  ```
+  $ rosrun nng_test publisher
+  ```
+  ```
+  $ rosrun nng_test subscriber
+  ```
+- Listen to received messages for the topic `/nnc_test/received_nng_subscription`.
+  ```
+  $ rostopic echo /nnc_test/received_nng_subscription
+  ```
+- Trigger a transmission by publishing a string message to `/nnc_test/publish_to_nng`.
+  ```
+  $ rostopic pub -1 /nnc_test/publish_to_nng std_msgs/String "Some Message"
+  ```
+  The `rostopic echo` command will show the received message.
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