plugin-feed: Note feed xml file is production only (#5305)
plugin-feed: Note feed xml file is production only (#5305)

Add note that the rss.xml feed is only generated in production.
0x80/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,fk/gatsby,0x80/gatsby,0x80/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,ChristopherBiscardi/gatsby