YAML mapping documentation of uniqueConstraint

I have provided an example of YAML mapping for unique constraints which is equivalent to the [annotation reference for unique constraints](http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/annotations-reference.html#annref-uniqueconstraint).

It is really basic, but I think better than nothing. Because there are severall people who search for that.

See
* http://stackoverflow.com/questions/8893473/how-to-add-a-constraint-index-in-yaml-doctrine-2
* http://stackoverflow.com/questions/7347581/how-to-create-a-unique-constraints-in-yml-in-doctrine2

I felt the need of that doc, too ;)