Updated migrating doc from v2/v3 to v4 (#6198)
Updated migrating doc from v2/v3 to v4 (#6198)

* Updated migrating doc from v2/v3 to v4

Added:
- optional parameters
- routeParams
- query - gone, only unparsed search
- getParamNames

* Fix some grammar and wording

reactjs/react-router,reactjs/react-router,ReactTraining/react-router,ReactTraining/react-router,rackt/react-router,rackt/react-router,ReactTraining/react-router,ReactTraining/react-router