Pelican plugin install method has been changed in version 4.5. So you can use plugin easier than before.
You should not download plugin files from github. It can use pip command for plugin downloads directoly and add plugin automatically.
Usage : pip install pelican.plugin name
Example for want to use more_categories plugin.
$ pip install pelican.more_categories
Now, you can add some categories to Metadata syntax.
article.md :
Category: foo, bar
