extra polyfills must be included manually (facebook#5814)

* extra polyfills must be included manually, fixes facebook#5795

* make note italicized

Author: ajwann

packages/react-app-polyfill/README.md

@@ -13,6 +13,8 @@ Each polyfill ensures the following language features are present:
 1. `Symbol` (a built-in object used by `for...of` syntax and friends)
 1. `Array.from` (a built-in static method used by array spread, i.e. `[...arr]`)
 
+*If you need more features, you must include them manually.*
+
 ### Usage
 
 First, install the package using Yarn or npm: