Add a note about the codemod on the lifecycle warnings post (#2219)

* Add a note about the codemod on the lifecycle warnings post

https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html

* missing fullstop.

* Update 2018-03-27-update-on-async-rendering.md

content/blog/2018-03-27-update-on-async-rendering.md

@@ -25,6 +25,15 @@ These lifecycle methods have often been misunderstood and subtly misused; furthe
 
 We maintain over 50,000 React components at Facebook, and we don't plan to rewrite them all immediately. We understand that migrations take time. We will take the gradual migration path along with everyone in the React community.
 
+If you don't have the time to migrate or test these components, we recommend running a ["codemod"](https://medium.com/@cpojer/effective-javascript-codemods-5a6686bb46fb) script that renames them automatically:
+
+```bash
+cd your_project
+npx react-codemod rename-unsafe-lifecycles
+```
+
+Learn more about this codemod on the [16.9.0 release post.](https://reactjs.org/blog/2019/08/08/react-v16.9.0.html#renaming-unsafe-lifecycle-methods) 
+
 ---
 
 ## Migrating from Legacy Lifecycles {#migrating-from-legacy-lifecycles}