Update README.md

Author: huangshuwei

Diff for: README.md

@@ -48,6 +48,7 @@ Based on vue2.x flexible table components.
 -   [x] [Cell Span](http://happy-coding-clans.github.io/vue-easytable/#/en/doc/table/cell-span)
 -   [x] [Cell Selection(keyboard operation)](http://happy-coding-clans.github.io/vue-easytable/#/en/doc/table/cell-selection)
 -   [x] [Cell Edit](http://happy-coding-clans.github.io/vue-easytable/#/en/doc/table/cell-edit)
+-   [x] [Contextmenu](http://happy-coding-clans.github.io/vue-easytable/#/en/doc/table/contextmenu)
 -   [x] [Cell Ellipsis](http://happy-coding-clans.github.io/vue-easytable/#/en/doc/table/cell-ellipsis)
 -   [x] [Row Radio](http://happy-coding-clans.github.io/vue-easytable/#/en/doc/table/row-radio)
 -   [x] [Row Checkbox](http://happy-coding-clans.github.io/vue-easytable/#/en/doc/table/row-checkbox)