Add documentation for mapping in gatsby-config (#4054)

* Add documentation for mapping

* Fix glitch in pasted code

* Update gatsby-config.md

* format

* Update gatsby-config.md

* Update gatsby-config.md

docs/docs/gatsby-config.md

@@ -35,7 +35,7 @@ This way you can store it in one place, and pull it whenever you need it. If you
 
 See a fuller description and sample usage in [Gatsby.js Tutorial Part Four](/tutorial/part-four/#data-in-gatsby).
 
-## plugins
+## Plugins
 
 Plugins are Node.js packages that implement Gatsby APIs. The config file accepts an array of plugins. Some plugins may need only to be listed by name, while others may take options (see the docs for individual plugins).
 
@@ -69,7 +69,7 @@ module.exports = {
 
 See more about [Adding a Path Prefix](/docs/path-prefix/).
 
-## polyfill
+## Polyfill
 
 Gatsby uses the ES6 Promise API. Because some browsers don't support this, Gatsby includes a Promise polyfill by default.
 
@@ -83,11 +83,63 @@ module.exports = {
 
 See more about [Browser Support](/docs/browser-support/#polyfills) in Gatsby.
 
-## mapping
+## Mapping node types
 
-TODO
+Gatsby includes an advanced feature that lets you create "mappings" between node types.
 
-## proxy
+For instance, imagine you have a multi-author markdown blog where you want to "link" from each blog post to the author information stored in a yaml file named `author.yaml`:
+
+```markdown
+---
+title: A blog post
+author: Kyle Mathews
+---
+
+A treatsie on the efficacy of bezoar for treating agricultural pesticide poisoning.
+```
+
+author.yaml
+
+```yaml
+- id: Kyle Mathews
+  bio: Founder @ GatsbyJS. Likes tech, reading/writing, founding things. Blogs at bricolage.io.
+  twitter: "@kylemathews"
+```
+
+You can map between the `author` field in `frontmatter` to the id in the `author.yaml` objects by adding to your `gatsby-config.js`:
+
+```javascript
+module.exports = {
+  plugins: [...],
+  mapping: {
+    "MarkdownRemark.frontmatter.author": `AuthorYaml`,
+  },
+}
+```
+
+Gatsby then uses this mapping when creating the GraphQL schema to enable you to query data from both sources:
+
+```graphql
+query BlogPost($slug: String!) {
+  markdownRemark(fields: { slug: { eq: $slug } }) {
+    html
+    fields {
+      slug
+    }
+    frontmatter {
+      title
+      author {
+        # This now links to the author object
+        id
+        bio
+        twitter
+      }
+    }
+  }
+}
+```
+
+## Proxy
 
 Setting the proxy config option will tell the development server to proxy any unknown requests to your specified server. For example: