Added missing section from tutorial

It's easy to miss the step to add your module to `plugin.hpp` and `plugin.cpp`. It says you need to do that in the output from the helper script, but it should be a step in the tutorial itself.

PluginDevelopmentTutorial.md

@@ -74,6 +74,23 @@ to the init() function in plugin.cpp.
 ```
 Open `MyModule.svg` with Inkscape, open the Layers panel, and hide the `components` layer to hide component placeholders.
 
+## Add the module to the plugin
+
+The `createmodule` command above adds your module to the `plugin.json` manifest, but doesn't enable the module until you add it to
+`plugin.hpp` and `plugin.cpp`.
+
+Uncomment the following line (at or near line 11) in `src/plugin.hpp`,
+
+```cpp
+extern Model *modelMyModule;
+```
+
+Then uncomment the line (also at or near line 11) in `src/plugin.cpp`,
+
+```cpp
+p->addModel(modelMyModule);
+```
+
 ## Implementing the DSP kernel
 
 Rack modules have four basic components, as we saw in the [Panel Guide](Panel.html).