Disable gh-page setup instruction if scripts.deploy has been added

[n3tr]

resolve #1221

Disable github page setup instruction if scripts.deploy has been added to package.json

No deploy script

> yarn run build
The build folder is ready to be deployed.
To publish it at http://myname.github.io/myapp, run:

  yarn add --dev gh-pages

Add the following script in your package.json.

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

Then run:

  yarn run deploy

✨  Done in 7.45s.

after added deploy

The project was built assuming it is hosted at /myapp/.
You can control this with the homepage field in your package.json.

The build folder is ready to be deployed.
To publish it at http://myname.github.io/myapp, run:

  yarn run deploy

✨  Done in 7.08s.

[gaearon]

LGTM!