Add a bit of important documentation

docs/keymap.md

@@ -15,6 +15,14 @@ The basics of what you'll need to get started are:
 - Assign a `Firmware` instance to a variable (ex. `keyboard = Firmware()` - note
   the parentheses)
 
+- Make sure this `Firmware` instance is actually run at the end of the file with
+  a block such as the following:
+
+```python
+if __name__ == '__main__':
+    keyboard.go()
+```
+
 - Assign pins and your diode orientation (only necessary on handwire keyboards),
   for example: