add note on statement ordering in batch query

Try to explain why statement ordering isn't preserved in batch query, and suggest workaround.
[details](https://github.com/doanduyhai/Achilles/wiki/Batch-Mode#statements-ordering)

```python
class MyMode(Model):
        id    = columns.Integer(primary_key=True)
        count = columns.Integer()
        text  = columns.Text()

    with BatchQuery() as b:
       MyModel.batch(b).create(id=1, count=2, text='123') 
       MyModel.batch(b).create(id=1, count=3, text='111')

    assert MyModel.objects(id=1).first().count == 3
    assert MyModel.objects(id=1).first().text  == '123'

This behavior is not very intuitive, we could adding a faq to explain it.