Someone helped me by showing me this link:
It keeps refering to sample plugins:
A sample plugin project that illustrates how to work with document files is available in the <%IDEA project directory%>/community/samples/textEditor directory. For more information, see Sample Text File Editor.
I've been googling for a while, I can't figure out where to get this /community directory. Do I need to download it somewhere? Can someone give me a link?