Mention core-icons import in docs

Author: robdodson

core-icon.html

@@ -29,7 +29,7 @@
 See [core-iconset](#core-iconset) and [core-iconset-svg](#core-iconset-svg) for more information about
 how to use a custom iconset.
 
-See [core-icons](#core-icons) for the default set of icons.
+See [core-icons](#core-icons) for the default set of icons. To use the default set of icons you'll need to include an import for `core-icons.html`.
 
 @group Polymer Core Elements
 @element core-icon