Added note about string ref API to strict mode section

Author: bvaughn

content/blog/2018-02-07-react-v-16-3.md

@@ -57,6 +57,11 @@ We are also adding a new static lifecycle, `getDerivedStateFromProps`, as a safe
 
 Although it is not possible for strict mode to catch all problems (e.g. certain types of mutation), it can help with many. If you see warnings in strict mode, those things will likely cause bugs for async rendering.
 
-In version 16.3, `StrictMode` helps with two things: identifying unsafe lifecycles and detecting unexpected side effects. Additional functionality will be added with future releases of React.
+In version 16.3, `StrictMode` helps with only a handful of things:
+* Identifying components with unsafe lifecycles
+* Identifying usage of the legacy string ref API
+* Detecting unexpected side effects
+
+Additional functionality will be added with future releases of React.
 
 [Learn more about the `StrictMode` component here.](#)