The basic structure is as follows:

plugins
+ plugin_name
  + data
  + docs
  + static
  + templates
  + __init__.py
  \ README.md

The static files will be made accessible automatically at the following location: /plugins/plugin_name/static/...

All *.md files in the docs directory will be visible in the help page. The README.md (if available) will be the first entry in the help page and might be used in the future as a description for plug-in browsing.