Added docs for average
Added docs for average

ChuyuHsu/ScalaFunctional,EntilZha/PyFunctional,lucidfrontier45/ScalaFunctional,EntilZha/PyFunctional,EntilZha/ScalaFunctional,EntilZha/ScalaFunctional,ChuyuHsu/ScalaFunctional,lucidfrontier45/ScalaFunctional