document / give example of allocating memory for array messages

[joshua-8]

    // allocate memory for the message
    controlMsg.data.capacity = 100;
    controlMsg.data.size = 0;
    controlMsg.data.data = (float*)malloc(controlMsg.data.capacity * sizeof(float));
    controlMsg.layout.data_offset = 0;
    controlMsg.layout.dim.capacity = 100;
    controlMsg.layout.dim.size = 0;
    controlMsg.layout.dim.data = (example_interfaces__msg__MultiArrayDimension*)malloc(controlMsg.layout.dim.capacity * sizeof(example_interfaces__msg__MultiArrayDimension));
    for (size_t i = 0; i < controlMsg.layout.dim.capacity; i++) {
        controlMsg.layout.dim.data[i].label.capacity = 20;
        controlMsg.layout.dim.data[i].label.size = 0;
        controlMsg.layout.dim.data[i].label.data = (char*)malloc(controlMsg.layout.dim.data[i].label.capacity * sizeof(char));
    }

    addSub(control, example_interfaces__msg__Float32MultiArray, "/rcm/control");