Docs: add note on how to enable/disable preview links individually.

Closes #2005.

README.md

@@ -270,6 +270,8 @@ Reveal.initialize({
 	hideAddressBar: true,
 
 	// Opens links in an iframe preview overlay
+	// Add `data-preview-link` and `data-preview-link="false"` to customise each link
+	// individually
 	previewLinks: false,
 
 	// Transition style

js/reveal.js

@@ -145,6 +145,8 @@
 			hideAddressBar: true,
 
 			// Opens links in an iframe preview overlay
+			// Add `data-preview-link` and `data-preview-link="false"` to customise each link
+			// individually
 			previewLinks: false,
 
 			// Exposes the reveal.js API through window.postMessage