Add post on the <dialog> element

Close #347

posts/dialog.md

@@ -0,0 +1,13 @@
+feature: <dialog>
+status: caution
+tags: polyfill
+kind: html
+polyfillurls:[dialog-polyfill](https://github.com/GoogleChrome/dialog-polyfill)
+
+The `<dialog>` element can be used to create popup dialogs or modal dialogs for web applications, and is part of [HTML 5.1](http://www.w3.org/html/wg/drafts/html/master/interactive-elements.html#the-dialog-element).
+In its modal mode, it blocks all access to content behind the front-most dialog.
+
+The majority of [Google's polyfill](https://github.com/GoogleChrome/dialog-polyfill) goes towards preventing the browser from gaining focus or accessing content behind the dialog.
+You should consider that the preventative measures taken by the polyfill are not exhaustive, as they are not implemented natively.
+
+Supported in Chrome 37+ and Opera 24+, [tracked](https://bugzilla.mozilla.org/show_bug.cgi?id=840640) for Firefox.