Add gatsby-node-helpers description to create-source-plugin.md (#3365)
Add gatsby-node-helpers description to create-source-plugin.md (#3365)


danielfarrell/gatsby,gatsbyjs/gatsby,fk/gatsby,fabrictech/gatsby,gatsbyjs/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby,fk/gatsby,Khaledgarbaya/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,Khaledgarbaya/gatsby,fabrictech/gatsby,Khaledgarbaya/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,fabrictech/gatsby,0x80/gatsby,0x80/gatsby,mingaldrichgan/gatsby,0x80/gatsby,mingaldrichgan/gatsby,danielfarrell/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,fk/gatsby