Add note on PropTypes

Author: m-allanson

packages/gatsby-transformer-react-docgen/README.md

@@ -24,6 +24,8 @@ You'll also need to include a source-plugin, such as
 [gatsby-source-filesystem](https://www.npmjs.com/package/gatsby-source-filesystem),
 so that the transformer has access to source data.
 
+Note that at least one of your React Components must have PropTypes defined.
+
 ## How to query
 
 An example _graphql_ query to get nodes: