[css-2015] Add issue for css-backgrounds cross-linking. Gonna need to bikeshed that spec soon.

Author: fantasai

css-2015/Overview.bs

@@ -899,6 +899,8 @@ ISSUE: Bikeshed should be amended shortly to allow auto-genning the propdef inde
 
 <h3 id="values">Values Index</h3>
 
+ISSUE: Cross-linking from CSS Backgrounds and Borders is kinda broken atm...
+
 <index type="value" status="TR" for="
 	css21,
 	css-syntax-3,