function create(req, res, next) {
  const user = new User({
    username: req.body.username,
    mobileNumber: req.body.mobileNumber
  });

  user.save()
    .then(savedUser => res.json(savedUser))
    .catch(e => next(e));
}
Create new user
@property {string} req.body.username - The username of user.
@property {string} req.body.mobileNumber - The mobileNumber of user.
@returns {User}