[gatsby-source-wordpress] add section about image processing to docs (#3120)
[gatsby-source-wordpress] add section about image processing to docs (#3120)


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