This plugin is useful only for developers, cause it provides only API, not any commands, configs etc.
To use its API you should copy jar into your plugins folder and type:
depend:
- QumassoScoreboards
into plugin.yml
Then you should add plugin as dependency in Maven:

<dependency>
<groupId>com.github.qumasso</groupId>
<artifactId>qumasso-scoreboards</artifactId>
<version>1.0-SNAPSHOT</version>
<scope>system</scope>
<systemPath>Path\to\plugin\qumasso-scoreboards-1.0-SNAPSHOT.jar</systemPath>
</dependency>

Or if you don't use Maven just add this jar as external library in your IDE.

Finally get the plugin instance like this:

ScoreboardsPlugin scoreboards = (ScoreboardsPlugin) getServer().getPluginManager().getPlugin("QumassoScoreboards");

Now you are ready to use some scoreboards API.