Disable "deploy" hint if "deploy" exists in package.json "scripts"

[gaearon]

We print the following for GH Pages homepage:

The build folder is ready to be deployed.
To publish it at https://xxx.github.io/yyy/, run:

  yarn add --dev gh-pages

Add the following script in your package.json.

    // ...
    "scripts": {
      // ...
      "deploy": "npm run build&&gh-pages -d build"
    }

I think we can skip the part about setting up deploy if you already have deploy in package.json.

[n3tr]

created PR #1222