add warning about blocking 3rd-party data to todomvc example

Author: chrisvfritz

src/examples/todomvc.md

@@ -6,4 +6,6 @@ order: 9
 
 > This is a fully spec-compliant TodoMVC implementation in under 120 effective lines of JavaScript (excluding comments and blank lines).
 
+<p class="tip">Note that if your web browser is configured to block 3rd-party data/cookies, the example below will not work, as the `localStorage` data will fail to be saved from JSFiddle. You'll have to click on `Edit in JSFiddle` to see the live result.</p>
+
 <iframe width="100%" height="500" src="https://jsfiddle.net/yyx990803/4dr2fLb7/embedded/result,html,js,css" allowfullscreen="allowfullscreen" frameborder="0"></iframe>