adding proptype to MessageSimple

Author: Vir Desai

src/components/MessageSimple/index.js

@@ -316,6 +316,8 @@ export const MessageSimple = themed(
       readOnly: PropTypes.bool,
       /** Disables the message UI. Which means, message actions, reactions won't work. */
       disabled: PropTypes.bool,
+      /** Object specifying rules defined within simple-markdown https://github.com/Khan/simple-markdown#adding-a-simple-extension */
+      markdownRules: PropTypes.object,
     };
 
     static defaultProps = {