the official library is here: https://github.com/AJS-development/OgarUL-Plugin-Library
To install any plugins from the library, first do plugin available and remember the name of the plugin you want. then do plugin install [name] and you are set!
To use a plugin, simply download the folder and drag it into the plugins folder in src or use the plugin add command. To use the plugin add command, the plugin must have a files.txt file. then click on that file, click raw, and then copy the url. Then do plugin add [url] [pluginname] and it will even reload automatically for you. Thats it! example, doing plugin add https://raw.githubusercontent.com/AJS-development/OgarUL-Plugin-Library/master/devtools-plugin/files.txt devtools will add a plugin called devtools to your plugins.
This is the fun part, creating your own plugins. There is an example plugin you should look at and there is a template plugin. The documentation is in the wiki at ogar unlimited