mention Rationale.md in README.md
mention Rationale.md in README.md

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