Add link to EcmaScript poposal

Signed-off-by: Rick Beerendonk <[email protected]>

Author: rickbeerendonk

03 - Properties - a. Default Values/stage2/jsx-es2015/app.jsx

@@ -5,6 +5,7 @@
 /* eslint react/prop-types:"off" */
 
 class Greeting extends React.Component {
+  // See: https://github.com/tc39/proposal-class-fields
   static defaultProps = {
     name: 'World'
   }