[css-2015] Start the 2015 snapshot, with at least the issue outlining what it needs to look like.

Author: tabatkins

css-2015/Overview.bs

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+<pre class='metadata'>
+Title: CSS Snapshot 2015
+Shortname: css-2015
+Level: 1
+Status: NOTE
+Group: CSSWG
+URL: https://dev.w3.org/csswg/css-2015/
+Editor: Tab Atkins Jr., Google, http://xanthir.com/
+Abstract: This snapshot summarizes the specs that are "stable" and broadly usable in CSS, as of 2015.
+</pre>
+
+Introduction {#intro}
+=====================
+
+<div class="issue">
+	* RESOLVED: Produce the snapshot with list below, update vendor prefix policy.
+		Group will review when finished.
+
+		* Everything in REC
+		* Animations L1
+		* Backgrounds and Borders L3,
+		* Cascading and Inheritance L3
+		* Compositing and Blending L1
+		* Conditional L3
+		* Flexible Box Layout L1,
+		* Fonts L3
+		* Images and Replaced Content L3
+		* Multicolumn Layout L1
+		* Syntax L3
+		* Transforms L1
+		* Transitions L1,
+		* Values and Units L3
+		* plus will-change and CSS3 UI once stable updates are published to TR
+	* Plan to incorporate intro material from CSS2.1 in next update
+	* Plan to incorporate property indices/glossaries/etc. once we can generate them for an entire snapshot
+</div>