[Docs] added initialVisibleMonth to DayPickerRangeController in ReadMe

Fixes #1619.

Author: AlokTakshak

README.md

@@ -296,6 +296,7 @@ Here is the minimum *REQUIRED* setup you need to get the `DayPickerRangeControll
   onDatesChange={({ startDate, endDate }) => this.setState({ startDate, endDate })} // PropTypes.func.isRequired,
   focusedInput={this.state.focusedInput} // PropTypes.oneOf([START_DATE, END_DATE]) or null,
   onFocusChange={focusedInput => this.setState({ focusedInput })} // PropTypes.func.isRequired,
+  initialVisibleMonth={() => moment().add(2, "M")} // PropTypes.func or null,
 />
 ```