Add description on how to disable mangling

[simmue]

As already mentioned here #27, please add a description on how to disable mangling for ionic 3 due to the following issue: typeorm/typeorm#2164
I lost a lot of time because of this issue.

Description could be something like this:
Disable mangling as described under the following link:
https://ionicframework.com/docs/v3/developer-resources/app-scripts/

1. Create a new file under ./config/uglifyjs.config
2. Copy paste the following content

module.exports = {
  /**
   * mangle: uglify 2's mangle option
   */
  mangle: false,

  /**
   * compress: uglify 2's compress option
   */
  compress: {
    toplevel: true,
    pure_getters: true
  }
};

3. Add the path to the file under package.json like this:

....
 "config": {
    "ionic_uglifyjs": "./config/uglifyjs.config.js"
  },
...