documentation improves on how this plugin works

[kratsg]

This threw me for a loop because I forgot how the plugin works, so perhaps some documentation could be added along the lines of:

"This plugin looks for files named by the filename specified in nav_file and replaces the default navigation with it, wherever it finds it."

(related: nav_file has to be a filename only, if it's folder/SUMMARY.md, it silently breaks?)