added Gatsby.js v2 webpack info to README.md (dfee#82)

README.md

@@ -125,7 +125,7 @@ resolve {
 
 ```
 
-For Gatsby.js you can add a `modifyWebpackConfig` export to your `gatsby-node.js` file:
+**For Gatsby.js v1** you can add a `modifyWebpackConfig` export to your `gatsby-node.js` file:
 
 ```
 exports.modifyWebpackConfig = ({config, env}) => {
@@ -140,6 +140,28 @@ exports.modifyWebpackConfig = ({config, env}) => {
 }
 ```
 
+**For Gatsby.js v2** you can add a `onCreateWebpackConfig` export to your `gatsby-node.js` file:
+
+```
+const path = require('path')
+
+exports.onCreateWebpackConfig = ({
+  stage,
+  getConfig,
+  rules,
+  loaders,
+  actions,
+}) => {
+  actions.setWebpackConfig({
+    resolve: {
+      alias: {
+        '_variables.sass': path.resolve(__dirname, 'relative/path/from/webpack/config/to/your/_variables.sass'),
+      },
+    },
+  })
+}
+```
+
 ### Override Bulma variables in CRA _without_ eject
 
 Sometimes, you don't want to eject your CRA (Create React App) application. But how to make custom style variables for Bulma? There is a simple and right way to solve it!