Clarify that if gatsby isn't installed you need to run `npm install` (#3272)
Clarify that if gatsby isn't installed you need to run `npm install` (#3272)


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