Update README.md (#9955)
Update README.md (#9955)

I'm adding this in response to #9436 which brings up some of the confusion in the community around gtag.js vs anlaytics.js this PR hopes to clarify some of this confusion at the top of the README where no one can miss it.
gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby