Mention the rfc to move targets top level in the readme?

[hzoo]

Using this makes it clear we don't want to have different sets of target options babel/rfcs#2

{
  "presets": [
    ["@babel/preset-env", {
      "targets": ">1%, not ie 11"
    }]
  ],
"plugins": [
    [
      "babel-plugin-polyfill-corejs3",
      {
        "method": "usage-global",
        "targets": {
          "edge": 16
        }
      }
    ]
  ]
}

[agilgur5]

I believe this was at least partially completed by 1694f0c , which added a link to the RFC in the Migration docs (which are mentioned in the README).

But in any case, the RFC has been implemented now and 1091fa4 changed the examples to use top-level targets, so I think this can almost certainly be closed now