.msg and .srv creation documented

src/custom_interfaces/Readme.md

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+Use this ROS2 package to create custom interfaces, eg. topic (*.msg) and/or service (*.srv) files.  
+This package must only be used for interface files and __no__ Python scripts.  
+
+The following steps describe how to create a new custom topic or service.   
+
+## 1. Create new interface file
+Topic *.msg files got into `msg` directory,  
+Service *.srv files got into `srv` directory.  
+
+Create these files according to the example on this [instructions page](https://docs.ros.org/en/foxy/Tutorials/Writing-A-Simple-Py-Service-And-Client.html).
+
+## 2. CMakeLists.txt
+Edit the `CMakeLists.txt` file by adding the newly created *.msg or *.srv files to this part:
+```cmake
+rosidl_generate_interfaces(${PROJECT_NAME}
+  "msg/CustomMsg1.msg"
+  "msg/CustomMsg2.msg"
+  "srv/CustomSrv1.srv"
+  DEPENDENCIES builtin_interfaces
+ )
+```
+
+## 3. Build colcon workspace
+Move back to the root of your _colcon_ workspace and rebuild it: `colcon build --symlink-install`  
+It should build without any errors. If you encounter errors fix these before continuing.
+
+After a successful build you must __close and reopen all terminals__ in order to source the newly built workspace. For this to happen you must have edited _.bashrc_ and added your current workspace (`sudo nano ~/.bashrc` and follow [this instruction](None)).
+
+## 4. Import interfaces
+Import the newly created interfaces into the Python scripts. These scripts must be located in _Python packages_ inside the same workspace (or another _sourced_ workspace):  
+* Topics:  
+```python
+from custom_interfaces.msg import CustomMsg1
+```
+
+* Services:  
+```python
+from custom_interfaces.srv import CustomSrv1
+```
+Now you can work with your custom interface in order to exchange custom information between nodes.