add basic documentation about publicURL field on File node (#3752)

* add basic documentation about copying files from File nodes to build directory

* A few copy edits

* Format

Author: pieh

docs/docs/adding-images-fonts-files.md

@@ -51,7 +51,58 @@ production so you don’t need to worry about long-term caching of assets.
 
 Please be advised that this is also a custom feature of Webpack.
 
-An alternative way of handling static assets is described in the next section.
+Two alternative ways of handling static assets is described in the next sections.
+
+## Query for `File` in GraphQL queries using gatsby-source-filesystem
+
+You can query the `publicURL` field of `File` nodes found in your data layer to trigger copying those files to the public directory and get URLs to them.
+
+Examples:
+
+* Copy all `.pdf` files you have in your data layer to your build directory and return URLs to them:
+
+  ```graphql
+  {
+    allFile(filter: { extension: { eq: "pdf" } }) {
+      edges {
+        node {
+          publicURL
+        }
+      }
+    }
+  }
+  ```
+
+* Copy post attachments defined in your Markdown files:
+
+  Link to your attachments in the markdown frontmatter:
+
+  ```markdown
+  ---
+  title: "Title of article"
+attachments:
+  - "./assets.zip"
+  - "./presentation.pdf"
+  ---
+
+  Hi, this is a great article.
+  ```
+
+  In the article template component file, you can query for the attachments:
+
+  ```graphql
+  query TemplateBlogPost($slug: String!) {
+    markdownRemark(fields: { slug: { eq: $slug } }) {
+      html
+      frontmatter {
+        title
+        attachments {
+          publicURL
+        }
+      }
+    }
+  }
+  ```
 
 ## Using the `static` Folder
 

packages/gatsby-source-filesystem/src/extend-file-node.js

@@ -11,6 +11,7 @@ module.exports = ({ type, getNodeAndSavePathDependency, pathPrefix = `` }) => {
     publicURL: {
       type: GraphQLString,
       args: {},
+      description: `Copy file to static directory and return public url to it`,
       resolve: (file, fieldArgs, context) => {
         const details = getNodeAndSavePathDependency(file.id, context.path)
         const fileName = `${file.name}-${file.internal.contentDigest}${