Completed instructions to use gatsby-transformer-excel (#4852)

* Completed instructions to use gatsby-transformer-excel

After trying to use the plugin for a while, I figured out that I needed to use `gatsby-source-filesystem` to point my Excel's files directory.

I propose to update the documentation in this way.

* Update README.md

packages/gatsby-transformer-excel/README.md

@@ -6,13 +6,30 @@ Parses Excel files into JSON arrays.
 
 `npm install --save gatsby-transformer-excel`
 
+Note: You generally will use this plugin together with the [`gatsby-source-filesystem`](/packages/gatsby-source-filesystem/) plugin. `gatsby-source-filesystem` reads in the files then this plugin *transforms* the files into data you can query.
+
 ## How to use
 
+If you put your Excel's files in `./src/data`:
+
 ```javascript
 // In your gatsby-config.js
-plugins: [`gatsby-transformer-excel`];
+module.exports = {
+  plugins: [ 
+    {
+      resolve: `gatsby-source-filesystem`,
+      options: {
+        name: `data`,
+        path: `${__dirname}/src/data/`,
+      },
+    },
+    `gatsby-transformer-excel`,
+  ],
+}
 ```
 
+You can see an example project at https://github.com/gatsbyjs/gatsby/tree/master/examples/using-excel.
+
 ## Parsing algorithm
 
 The parsing is powered by the [SheetJS / js-xlsx](https://git.io/xlsx) library.