Clarify prod differences

README.md

@@ -25,11 +25,11 @@ If you prefer a `<script>` tag, you can get it from `window.PropTypes` global:
 <script src="https://unpkg.com/prop-types/prop-types.min.js"></script>
 ```
 
-## Development and Production Versions
+## Difference from `React.PropTypes`: Development and Production Versions
 
-In production, all validator functions are replaced with empty functions that throw an error. This is done to optimize the bundle size.
+In production, **all validator functions are replaced with empty functions that throw an error**. This is done to optimize the bundle size. This is new behavior, and you will only encounter it when you migrate from `React.PropTypes` to the `prop-types` package. For the vast majority of components, this doesn’t matter, and if you didn’t see [this warning](https://facebook.github.io/react/warnings/dont-call-proptypes.html) in your components, your code is safe to migrate. This is not a breaking change in React because you are only opting into this change for a component by explicitly changing your imports to use `prop-types`.
 
-Don’t call the validator functions manually in your code. React automatically calls `PropTypes` validators declared on your components in development version, and it won’t call them in production.
+Don’t call the validator functions manually in your code. React automatically calls `PropTypes` validators declared on your components in development version, and it won’t call them in production. **If you absolutely need to trigger the validation manually**, call `PropTypes.checkPropTypes(propTypes, props, location, componentName)`. Unlike the validators themselves, this function is safe to call in production, as it will be replaced by an empty function.
 
 If you use a bundler like Browserify or Webpack, don’t forget to [follow these instructions](https://facebook.github.io/react/docs/installation.html#development-and-production-versions) to correctly bundle your application in development or production mode. Otherwise you’ll ship unnecessary code to your users.
 
@@ -39,4 +39,4 @@ Refer to the [React documentation](https://facebook.github.io/react/docs/typeche
 
 ## Migrating from React.PropTypes
 
-Check out [Migrating from React.PropTypes](https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html#migrating-from-react.proptypes) for details on how to migrate to `prop-types` from `React.PropTypes`
+Check out [Migrating from React.PropTypes](https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html#migrating-from-react.proptypes) for details on how to migrate to `prop-types` from `React.PropTypes`.