docs: Add examples of using gutter (#66)

To help people avoid getting this error by using a number:
Warning: Failed prop type: Invalid prop `gutter` of type `number` supplied to `Masonry`, expected `string`.

Also to illustrate how to have responsive gutters because this was not obvious, see:
cedricdelpoux/react-responsive-masonry#6 (comment)

README.md

@@ -81,10 +81,10 @@ class MyWrapper extends Component {
 
 ### Mansonry component
 
-| Name         | PropType | Description                   | Default |
-| ------------ | -------- | ----------------------------- | ------- |
-| columnsCount | Number   | Injected by ResponsiveMasonry | 3       |
-| gutter       | String   | Margin surrounding each item  | "0"     |
+| Name         | PropType | Description                                           | Default |
+| ------------ | -------- | ----------------------------------------------------- | ------- |
+| columnsCount | Number   | Injected by ResponsiveMasonry                         | 3       |
+| gutter       | String   | Margin surrounding each item e.g. "10px" or "1.5rem"  | "0"     |
 
 ### ResponsiveMasonry component