Link to web component support

Author: dgraham

README.md

@@ -33,12 +33,17 @@ import '@github/markdown-toolbar-element'
 
 ## Browser support
 
+Browsers without native [custom element support][support] require a [polyfill][].
+
 - Chrome
 - Firefox
-- Safari 9+
+- Safari
 - Internet Explorer 11
 - Microsoft Edge
 
+[support]: https://caniuse.com/#feat=custom-elementsv1
+[polyfill]: https://github.com/webcomponents/custom-elements
+
 ## Development
 
 ```