Add documentation for mapping

Author: ajayns

docs/docs/gatsby-config.md

@@ -85,7 +85,147 @@ See more about [Browser Support](/docs/browser-support/#polyfills) in Gatsby.
 
 ## mapping
 
-TODO
+---
+title: Gatsby Config
+---
+
+Site configuration options for a Gatsby site are placed in a file at the root of the project folder called `gatsby-config.js`.
+
+_Note: There are many sample configs which may be helpful to reference in the different [Gatsby Example Websites](https://github.com/gatsbyjs/gatsby/tree/master/examples)._
+
+## Configuration options
+
+Options available to set within `gatsby-config.js` include:
+
+1. siteMetadata (object)
+2. plugins (array)
+3. pathPrefix (string)
+4. polyfill (boolean)
+5. mapping (object)
+6. proxy (object)
+
+## siteMetadata
+
+When you want to reuse common pieces of data across the site (for example, your site title), you can store that data in `siteMetadata`:
+
+```javascript
+module.exports = {
+  siteMetadata: {
+    title: `Gatsby`,
+    siteUrl: `https://www.gatsbyjs.org`,
+    description: `Blazing-fast static site generator for React`,
+  },
+};
+```
+
+This way you can store it in one place, and pull it whenever you need it. If you ever need to update the info, you only have to change it here.
+
+See a fuller description and sample usage in [Gatsby.js Tutorial Part Four](/tutorial/part-four/#data-in-gatsby).
+
+## 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).
+
+```javascript
+module.exports = {
+  plugins: [
+    `gatsby-plugin-react-helmet`,
+    {
+      resolve: `gatsby-source-filesystem`,
+      options: {
+        name: `docs`,
+        path: `${__dirname}/../docs/`,
+      },
+    },
+  ],
+};
+```
+
+See more about [Plugins](/docs/plugins/) for more on utilizing plugins, and to see available official and community plugins.
+
+## pathPrefix
+
+It's common for sites to be hosted somewhere other than the root of their domain. Say we have a Gatsby site at `example.com/blog/`. In this case, we would need a prefix (`/blog`) added to all paths on the site.
+
+```javascript
+module.exports = {
+  // Note: it must *not* have a trailing slash.
+  pathPrefix: `/blog`,
+};
+```
+
+See more about [Adding a Path Prefix](/docs/path-prefix/).
+
+## polyfill
+
+Gatsby uses the ES6 Promise API. Because some browsers don't support this, Gatsby includes a Promise polyfill by default.
+
+If you'd like to provide your own Promise polyfill, you can set `polyfill` to false.
+
+```javascript
+module.exports = {
+  polyfill: false,
+};
+```
+
+See more about [Browser Support](/docs/browser-support/#polyfills) in Gatsby.
+
+## mapping
+
+To query between nodes, Gatsby has a mapping feature which allows you to link two different nodes by id and then you can query with GraphQL. For instance, if you have a couple of blog posts which have author id in the frontmatter:
+
+```
+title: A blog post
+author: Kyle Mathews
+```
+
+And you have a list of authors and their details stored in `authors.yaml`, you can map between `author` in `frontmatter` to id in `authors.yaml` file by:
+
+```
+module.exports = {
+  mapping: {
+    "MarkdownRemark.frontmatter.author": `AuthorYaml`,
+  },
+}
+```
+
+This enables you to query data from both sources together:
+
+```
+query BlogPost($slug: String!) {
+    markdownRemark(fields: {slug: {eq: $slug}}) {
+        html
+        fields {
+            slug
+        }
+        frontmatter {
+            title
+            author {
+                id
+                fields {
+                    slug
+                }
+            }
+        }
+    }
+}
+```
+
+## proxy
+
+Setting the proxy config option will tell the development server to proxy any unknown requests to your specified server. For example:
+
+```javascript
+module.exports = {
+  proxy: {
+    prefix: "/api",
+    url: "http://examplesite.com/api/",
+  },
+};
+```
+
+See more about [Proxying API Requests in Development](/docs/api-proxy/).
+
 
 ## proxy