docs: Add instructions on how to add new functionality (#73)

This change adds documentation on how to integrate new functionality to
the ros bridge.
It aims to provide enough information to enable new contributors to
integrate the features they need on their own.

README.md

@@ -88,6 +88,53 @@ The bridge can also be started via ros run, therefore use the following instruct
 run `cargo ament-build --install-base install/voraus-ros-bridge -- --release`.
 Then `ros2 run voraus-ros-bridge voraus-ros-bridge`
 
+### How to add new voraus.core functionality to ROS
+
+The voraus.core has an OPC UA interface which this bridge utilizes.
+Hence, the first step to expose new voraus.core functionality to the ROS 2 world is to validate that the desired functionality is available within the OPC UA interface (right now this must be done by inspecting the OPC UA server via tools like [UaExpert](https://www.unified-automation.com/products/development-tools/uaexpert.html) or the [Python OPC UA client](https://github.com/FreeOpcUa/opcua-client-gui)).
+Then, the OPC UA functionality must be mapped to an adequate ROS 2 equivalent.
+For OPC UA variables most of the time this is a ROS topic but could theoretically also be a getter ROS service.
+OPC UA methods should be mapped to ROS services but it could also be convenient to use ROS subscriptions (see the `impedance_control/set_wrench` topic).
+
+To showcase the implementation of a new feature, the voraus.core `move_joints` functionality is used as an example in the following.
+According to the voraus.core OPC UA specification, the `move_joints` method can be reached at `ns=1;i=100211` with the parent node at `ns=1;i=100210`.
+The method has quiet a lot of arguments, so it's most convenient to call it via a ROS service.
+
+A ROS service is specified by its input and output arguments separated by `---` in a .srv file.
+Hence, create a new file called `MoveJoints.srv` within `voraus_interfaces/srv` containing the following:
+
+```
+bool relative
+uint32 target_reference_cs
+float64[] target_coordinate
+uint32 arriving_cs
+float64 velocity_scaling
+bool with_blending
+float64 blending_parameter
+int32[] config_vector
+uint32 command_id
+bool manual_mode
+---
+```
+
+After that, add the new file to the `CMakeLists.txt` of the `voraus_interfaces` package and compile it via `colcon build`.
+As a result, you are now able to use the new service type within the rust project.
+
+The next step is to add a new method to the `ROSServices` struct, which will be later used as a callback for the ROS service call.
+This method defines the OPC UA object id and method id of the desired voraus.core function.
+It is additionally responsible to call the OPC UA method with the correct arguments provided by the service request.
+
+Next, create a new ROS service in the `main.rs` and register the previously created method as a callback.
+
+The new method can be added in the `test_bridge_methods` module in order to assert that it is properly propagated.
+Run the integration test to verify the new test gets executed and passes.
+Lastly, verify that it actually works E2E by starting a voraus.core and test the new functionality.
+
+> **Note**  
+Don't forget to add the new functionality to the README's State of Integration section.
+
+For the concrete implementation of this example see this [pull request](https://github.com/vorausrobotik/voraus-ros-bridge/pull/74) (Note that the implementation details will likely change over time).
+
 ### Custom message/service files
 
 Create a separate ROS message package just like you would in cpp.