Update README.md (#3216)
Update README.md (#3216)

Discovered that the sizeByPixelDensity being set to 'true' by default was sizing my images to 39px.
fk/gatsby,danielfarrell/gatsby,mingaldrichgan/gatsby,mingaldrichgan/gatsby,fabrictech/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,danielfarrell/gatsby,mingaldrichgan/gatsby,0x80/gatsby,Khaledgarbaya/gatsby,Khaledgarbaya/gatsby,fabrictech/gatsby,fk/gatsby,Khaledgarbaya/gatsby,gatsbyjs/gatsby,fabrictech/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby