Update README.md with example of setting options

[aik099]

The #183 added ability to set Zombie options, when constructing the driver, but no example was added to the README.md. I think we should have one.

[aik099]

This is relevant code line: https://github.com/minkphp/MinkZombieDriver/blob/master/src/NodeJS/Server.php#L86

Basically specifying $options array during ZombieServer class object creation would allow to set any of Zombie supported options, including proxy.

Actual supported options at listed in Zombie code: https://github.com/assaf/zombie/blob/master/src/index.js#L33-L35

CC: @historylife