Add note regarding location of '/___graphql' (#1728)
Add note regarding location of '/___graphql' (#1728)

* Add note regarding location of '/___graphql'

* Tweak copy

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