Added dragSelectionEnabled to README

I used the "drag n drop" terminology from the top of the page, but feel free to change it if needed.

Author: trabianmatt

README.md

@@ -124,6 +124,7 @@ focusedRange(DateRange)              | Object    |                  | It defines
 onRangeFocusChange(DateRange)        | Object    |                  | Callback function for focus changes
 preview(DateRange)                   | Object    |              | displays a preview range and overwrite DateRange's default preview. Expected shape: `{ startDate: Date, endDate: Date, color: String }`
 showPreview(DateRange)               | bool      | true             | visibility of preview
+dragSelectionEnabled(Calendar)       | bool      | true             | whether dates can be selected via drag n drop
 onPreviewChange(DateRange)           | Object    |                  | Callback function for preview changes
 dateDisplayFormat(DateRange)         | String    | `MMM D, YYYY`    | selected range preview formatter. Check out [date-fns's format option](https://date-fns.org/v2.0.0-alpha.7/docs/format)
 staticRanges(`DefinedRange`, `DateRangePicker`) | Array   | [default preDefined ranges](https://github.com/Adphorus/react-date-range/blob/master/src/defaultRanges.js)             | -