Adds add'l docs to gatsby-source-filesystem

packages/gatsby-source-filesystem/README.md

@@ -58,3 +58,65 @@ You can query file nodes like the following:
   }
 }
 ```
+
+## Helper functions
+
+`gatsby-source-filesystem` exports two helper functions:
+
+* `createFilePath`
+* `createRemoteFileNode`
+
+### createFilePath
+
+When building pages from files, you often want to create a URL from a file's path on the file system. E.g. if you have a markdown file at `src/content/2018-01-23-an-exploration-of-the-nature-of-reality/index.md`, you might want to turn that into a page on your site at `example.com/2018-01-23-an-exploration-of-the-nature-of-reality/`. `createFilePath` is a helper function to make this task easier.
+
+```javascript
+createFilePath({
+  // The node you'd like to convert to a path
+  // e.g. froom a markdown, JSON, YAML file, etc
+  node:
+  // Method used to get a node
+  // The parameter from `onCreateNode` should be passed in here
+  getNode:
+  // The base path for your files.
+  // Defaults to `src/pages`. For the example above, you'd use `src/contents`.
+  basePath:
+  // Whether you want your file paths to contain a trailing `/` slash
+  // Defaults to true
+  trailingSlash:
+})
+```
+
+#### Example usage
+
+The following is taken from [Gatsby Tutorial, Part Four](https://www.gatsbyjs.org/tutorial/part-four/#programmatically-creating-pages-from-data) and is used to create URL slugs for markdown pages.
+
+```javascript
+const { createFilePath } = require(`gatsby-source-filesystem`);
+
+exports.onCreateNode = ({ node, getNode, boundActionCreators }) => {
+  const { createNodeField } = boundActionCreators;
+  // Ensures we are processing only markdown files
+  if (node.internal.type === "MarkdownRemark") {
+    // Use `createFilePath` to turn markdown files in our `data/faqs` directory into `/faqs/slug`
+    const relativeFilePath = createFilePath({
+      node,
+      getNode,
+      basePath: "data/faqs/",
+    });
+
+    // Creates new query'able field with name of 'slug'
+    createNodeField({
+      node,
+      name: "slug",
+      value: `/faqs${relativeFilePath}`,
+    });
+  }
+};
+```
+
+### createRemoteFileNode
+
+```javascript
+TO DO
+```