Add a tutorial about using the new createTypes API

[eojthebrave]

There's a new createTypes API coming in a future version of Gatsby https://www.gatsbyjs.org/blog/2019-03-04-new-schema-customization/

Which could be used to allow you to write code in your gatsby-node.js file that could tell Gatsby that a particular field(s) from Drupal contain Markdown instead of text. Allowing you to use all the gatsby-remark-* plugins. Probably not super useful for our example application, but could be useful for anyone that's building just static pages.