improve proxy property warning

src/core/instance/proxy.js

@@ -21,7 +21,8 @@ if (process.env.NODE_ENV !== 'production') {
       const isAllowedGlobal = allowedGlobals(key)
       if (!has && !isAllowedGlobal) {
         warn(
-          `Trying to access non-existent property "${key}" while rendering.`,
+          `Trying to access non-existent property "${key}" while rendering. ` +
+          `Make sure to declare reactive data properties in the data option.`,
           target
         )
       }