Mention custom parsers in the readme

[LinusU]

From #106

The latest version of standard (3.12.0) now supports custom JS parsers!

To use a custom parser, install it from npm (example: npm install babel-eslint) and add this to your package.json:

{
  "standard": {
    "parser": "babel-eslint"
  }
}

It would be good if this could be documented in the README.md

[yoshuawuyts]

PRs are welcome!

[feross]

Done.

[LinusU]

👍