Checkbox: Updating the readme to point to latest spec. (#2232)

* Updating the readme to point to latest spec

* Update packages/playground/src/components/Checkbox/README.md

Co-Authored-By: Makoto Morimoto <[email protected]>

Co-authored-by: Makoto Morimoto <[email protected]>

Author: aneeshack4

.github/CONTRIBUTING.md

@@ -3,6 +3,7 @@
 <!-- START doctoc generated TOC please keep comment here to allow auto update -->
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 
+
 - [Getting started](#getting-started)
   - [Useful Commands](#useful-commands)
 - [Workflow](#workflow)