Add note on extraHeight prop (#50)

Thanks @skierkowski!

Author: Maciej Skierkowski

README.md

@@ -95,7 +95,7 @@ All the `ScrollView`/`ListView` props will be passed.
 | `viewIsInsideTabBar` | `boolean` | Adds an extra offset that represents the `TabBarIOS` height. |
 | `resetScrollToCoords` | `Object: {x: number, y: number}` | Coordinates that will be used to reset the scroll when the keyboard hides. |
 | `enableAutoAutomaticScroll` | `boolean` | When focus in TextInput will scroll the position, default is enabled. |
-
+| `extraHeight` | `number` | Adds an extra offset |
 
 ## License