markdown module doc script: write how to generate the modules documentation

.gitignore

@@ -77,8 +77,10 @@ posix-configs/SITL/init/test/*_generated
 parameters.md
 airframes.md
 airframes.xml
+/modules
 
 cppcheck-result.xml
 cppcheck
 
 scan-build
+

Tools/px4moduledoc/markdownout.py

@@ -22,6 +22,12 @@ available commands, and in most cases `command help` will print the usage.
 
 Since this is generated from source, errors must be reported/fixed
 in the [Firmware](https://github.com/PX4/Firmware) repository.
+The documentation pages can be generated by running the following command from
+the root of the Firmware directory:
+```
+make module_documentation
+```
+The generated files will be written to the `modules` directory.
 
 ## Categories
 """