Update README.md to new constructors
Update README.md to new constructors

zhongdj/RxScala,jbripley/RxScala,jbripley/RxScala,zjrstar/RxScala,samuelgruetter/RxScala,hermanbanken/RxScala,ReactiveX/RxScala,TracyLu/RxScala,hermanbanken/RxScala,joohnnie/RxScala,samuelgruetter/RxScala