improve attr interpolation warning

src/compiler/parser/index.js

@@ -400,7 +400,8 @@ function processAttrs (el) {
           warn(
             `${name}="${value}": ` +
             'Interpolation inside attributes has been deprecated. ' +
-            'Use v-bind or the colon shorthand instead.'
+            'Use v-bind or the colon shorthand instead. For example, ' +
+            'instead of <div id="{{ val }}">, use <div :id="val">.'
           )
         }
       }