Add documentation for mapping in gatsby-config

docs/docs/gatsby-config.md

@@ -85,7 +85,44 @@ See more about [Browser Support](/docs/browser-support/#polyfills) in Gatsby.
 
 ## mapping
 
-TODO
+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