Add onChange handler to documentation

Author: bibekg

README.md

@@ -41,4 +41,5 @@ class App extends React.Component {
 `availability` | `Array<Date>` | List of dates that should be filled in on the grid (reflect the start time of each cell) | yes | N/A
 `numDays` | `number` | The number of days from today to show | no | `7`
 `minTime` | `number` | The minimum hour to show (0-23) | no | `9`
-`maxTime` | `number` | The maximum hour to show (0-23) | no | `23`
+`maxTime` | `number` | The maximum hour to show (0-23) | no | `23`
+`onChange` | `(Array<Date>) => void` | A callback that's executed when the selected availability is changed | yes | N/A