Update site-showcase-submissions.md concerning featured sites (#6281)
Update site-showcase-submissions.md concerning featured sites (#6281)


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