Update 1.0.3 Add PlayersAFK placeholder

Added a new placeholder, PlayersAFK, which outputs the current count of players who are AFK
Added maven repo to the pom.xml so that it can be compiled by other people without needing to also compile AFKPlus
Added a license to the repo, thanks to @NotAlexNoyle for providing a pull request for that
Added a readme which contains all the placeholders provided by this plugin and a description of each
Updated to Java 17 to match AFKPlus

The plugin name has also been updated to be properly cased, which will mean a new plugin folder

This shouldn't be much of an issue since the config had to be regenerated for new values anyway.