added MTurk update to NEWS.md
<h2>News</h2>
+2017-06-21: [MTurk now supports multiple assignments per HIT](https://github.com/facebookresearch/ParlAI/pull/156)
+
2017-06-16: [added Creating a New Task tutorial](http://parl.ai/static/docs/task_tutorial.html)
2017-05-31: [added Seq2Seq model](https://github.com/facebookresearch/ParlAI/pull/96)
