mavlink_system must be declared in global scope (#223)

Author: hamishwillee

en/mavgen_c/README.md

@@ -52,6 +52,15 @@ When compiling the project, we recommend that you specify the top-level output d
 $ gcc ... -I generated/include -I generated/include/common ...
 ```
 
+In order to *send messages* you will also need to declare a variable `mavlink_system` in the **global scope**, specifying the system id and component ID of your component:
+```cpp
+mavlink_system_t mavlink_system = {
+	1, // System ID (1-255)
+	1  // Component ID (a MAV_COMPONENT value)
+}; 
+```
+
+
 ## Multiple Streams ("channels") {#channels}
 
 The C MAVLink library utilizes a "channel" metaphor to allow for simultaneous processing of multiple, independent MAVLink streams in the same program.
@@ -286,4 +295,4 @@ The following examples show the use of the API.
 In addition, the C library is used in numerous open source systems:
 - [PX4](https://github.com/PX4/Firmware/blob/master/src/modules/mavlink/mavlink_receiver.cpp)
 - [ArduPilot](https://github.com/ArduPilot/ardupilot/blob/master/libraries/GCS_MAVLink/GCS_Common.cpp)
-- [MAVSDK](https://github.com/mavlink/MAVSDK/blob/develop/src/core/mavlink_receiver.cpp#L28-L51)
+- [MAVSDK](https://github.com/mavlink/MAVSDK/blob/develop/src/core/mavlink_receiver.cpp#L28-L51)