Reactant core is a spigot plugin framework which provide a better way for developing spigot plugins.
If you would like to compile it yourself, clone the project and ensure you have JDK 8.
After that, use ./gradlew build (gradlew build for windows) to start your build.
You can find the compiled jars in ./build/libs.
Use Reactant-*-all.jar which including the dependencies if you would like to load as spigot plugin.