Document how to create custom fragments (#3914)
Document how to create custom fragments (#3914)

* Document how to create custom fragments

* Fix ordering

* Add example component exporting a fragment

* Tweak language

gatsbyjs/gatsby,gatsbyjs/gatsby,0x80/gatsby,0x80/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,fk/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby