Added Formik in the list of libraries, which use render props (#2287)

Author: iamparnab

content/docs/render-props.md

@@ -14,7 +14,7 @@ A component with a render prop takes a function that returns a React element and
 )}/>
 ```
 
-Libraries that use render props include [React Router](https://reacttraining.com/react-router/web/api/Route/render-func) and [Downshift](https://github.com/paypal/downshift).
+Libraries that use render props include [React Router](https://reacttraining.com/react-router/web/api/Route/render-func), [Downshift](https://github.com/paypal/downshift) and [Formik](https://github.com/jaredpalmer/formik).
 
 In this document, we’ll discuss why render props are useful, and how to write your own.