Requires gatsby-source-filesystem in gatsby-config.js

[Mappboy]

Hi Stephen,

Might just want to update the readme that this requires gatsby-source-filesystem be in the gatsby-config.
Without it I was getting.

Error: Could not find owner plugin of node for loadNodeContent with owner `gatsby-source-filesystem`

It's pretty clear the cause, but I am assuming something bad happens if you change the name here ?

// Then create the node, as if it were created by the gatsby-source
// filesystem plugin.
return createNode(fileNode, {
        name: `gatsby-source-filesystem`
      });

Working fine now.

Thanks