Add an example on how to use the onEnter callback
Add an example on how to use the onEnter callback

This change adds an example on how to use the onEnter callback to redirect a user to another page.
d-oliveros/react-router,asaf/react-router,OpenGov/react-router,shunitoh/react-router,goblortikus/react-router,OpenGov/react-router,asaf/react-router,reactjs/react-router,d-oliveros/react-router,goblortikus/react-router,OpenGov/react-router,asaf/react-router,DelvarWorld/react-router,DelvarWorld/react-router,tylermcginnis/react-router,goblortikus/react-router,shunitoh/react-router,stshort/react-router,ReactTraining/react-router,aaron-goshine/react-router,ReactTraining/react-router,reactjs/react-router,tylermcginnis/react-router,ReactTraining/react-router,djkirby/react-router,rackt/react-router,rafrex/react-router,ReactTraining/react-router,aaron-goshine/react-router,stshort/react-router,rackt/react-router,OpenGov/react-router,goblortikus/react-router,djkirby/react-router,rafrex/react-router