Fix usage instructions for remark-embed-snippet (#4566)
Fix usage instructions for remark-embed-snippet (#4566)

* Fix usage instructions for remark-embed-snippet

It didn't mention that it needed to be in the `gatsby-transformer-remak`
plugins section.

* Update README.md

ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,0x80/gatsby,0x80/gatsby,mingaldrichgan/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,fk/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby