Update README.md (#3822)

add module.exports to How to use

Author: html5cat

packages/gatsby-transformer-sharp/README.md

@@ -16,7 +16,9 @@ including resizing, cropping, and creating responsive images.
 
 ```javascript
 // In your gatsby-config.js
-plugins: [`gatsby-transformer-sharp`];
+module.exports = {
+  plugins: [`gatsby-transformer-sharp`]
+}
 ```
 
 ## Parsing algorithm