Update README.md regarding babel [ci skip]
Update README.md regarding babel [ci skip]

Updates the readme to describe the new behavior of Babel and Gatsby.

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