Add headerinstructions

Header_Error.md

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+# Header Message Errors
+When wanting to introduce [headers](http://docs.ros.org/en/melodic/api/std_msgs/html/msg/Header.html) do the following steps.  
+  
+You should have a CMake package created already (as only Cmake packages can implement custom msgs as of now).
+
+## Custom Message File
+* Create a *.mgs file inside the `msg/` directory and start the file name with a capital letter
+* Add the line `std_msgs/Header header` in order to use the header message, this msg type is from standard messages ([std_msgs](http://docs.ros.org/en/melodic/api/std_msgs/html/index-msg.html)) 
+* Only introduce absolutely necessary comments in *.msg file (eg. #Header) in order to minimize possible errors.
+
+## Configure package.xml
+* Add _buildtool_depend_:  
+```xml
+<buildtool_depend>ament_cmake</buildtool_depend>
+<buildtool_depend>rosidl_default_generators</buildtool_depend>
+```
+* Add _build_depend_:  
+```xml
+<build_depend>builtin_interfaces</build_depend>
+<build_depend>std_msgs</build_depend>
+```
+* Add _exec_depend_:  
+```xml
+<exec_depend>rosidl_default_runtime</exec_depend>
+<exec_depend>builtin_interfaces</exec_depend>
+<exec_depend>std_msgs</exec_depend>
+```
+
+## Configure CMakeLists.txt
+* Add required packages:  
+```cmake
+find_package(ament_cmake REQUIRED)
+find_package(rosidl_default_generators REQUIRED)
+find_package(builtin_interfaces REQUIRED)
+find_package(std_msgs REQUIRED)
+```
+* Create msg files set (exchange with your own `msg/*.msg` files):  
+```cmake
+set(msg_files
+  "msg/IMUInfo.msg"
+  "msg/Extrinsics.msg"
+)
+```
+* Load set into _rosidl_generate_interfaces_ and add the dependency for std_msgs:  
+```cmake
+rosidl_generate_interfaces(${PROJECT_NAME}
+  ${msg_files}
+  DEPENDENCIES builtin_interfaces std_msgs
+)
+```
+
+## Build Colcon Workspace
+* Move to root of workspace: `cd ~/<path_to_ws>`
+* Run build: `colcon build`
+
+## Test and Verify
+* Run publisher: `ros2 topic pub /chatter <pkg_name>/msg/<custom_msg_file>`
+* Run subscriber: `ros2 topic echo /chatter`  
+  
+You should see your custom msg being published and received, including the header.
+
+## Source
+[Link to the problem's solution at ros.org forum](https://answers.ros.org/question/326008/ros2-run-symbol-not-found-on-custom-msg/)  
+Example Files:
+* ROS2 Realsense: 
+  * [Custom msg file](https://github.com/IntelRealSense/realsense-ros/blob/eloquent/realsense2_camera_msgs/msg/IMUInfo.msg)
+  * [CMakeLists.txt](https://github.com/IntelRealSense/realsense-ros/blob/eloquent/realsense2_camera_msgs/CMakeLists.txt)
+  * [package.xml](https://github.com/IntelRealSense/realsense-ros/blob/eloquent/realsense2_camera_msgs/package.xml)
+  * [Write timestamp](https://answers.ros.org/question/354203/timestamp-message-ros2-python/)