A basic plugin/addon for Kicad 9.0 that shows a Hello World popup.
Install the plugin via the Plugin and Content manager in Kicad. Click on the bottom left on "install from file" and open the hello_world.zip file. After installing, open the PCB editor and you will see a black box icon in the toolbar.
I found it extremely difficult to find a simple plugin on which I could build further in Kicad 8.0 and higher. So, after some research, I made my own "Hello World" plugin on which I can build further.
You can edit the files in ./plugins to create the plugin you want. When done, make a .zip file combining the plugin folder, the resources folder and the metadata.json file.