Document the usage of [populate](https://github.com/IllinoisCogComp/saul/blob/2481b77b40b6f495c141d701add8c7c4c165604c/saul-core/src/main/scala/edu/illinois/cs/cogcomp/saul/datamodel/node/Node.scala#L140) method: Also include details like: - what join nodes mean? - what are the implications of using it (costs vs benefits)
