Add guidance about using new ES6 features in docs

CONTRIBUTING.md

@@ -42,6 +42,12 @@ Ignore this if you're specifically describing `createElement`.
 
 Ignore this if you're specifically writing about ES5.
 
+**Don't use ES6 features when equivalent ES5 features have no downsides.**
+
+Remember that ES6 is still new to a lot of people. While we use it in many places (`const` / `let`, classes, arrow functions), if the equivalent ES5 code is just as straightforward and readable, consider using it.
+
+In particular, you should prefer named `function` declarations over `const myFunction = () => ...` arrows for top-level functions. However, you *should* use arrow functions where they provide a tangible improvement (such as preserving `this` context inside a component). Consider both sides of the tradeoff when deciding whether to use a new feature.
+
 **Don't use features that aren't standardized yet.**
 
 For example, **don't** write this: