Created Unassigned: samples for $ref [1669]

A sample of getting entity reference link(s) with attribute route:
```
Example 24: URL addressing the references between Categories(1) and Products
http://host/service/Categories(1)/Products/$ref
```

Another sample of deleting entity reference link using the following syntax with attribute route:
```
Example 25: two ways of unrelating Categories(1) and Products(0)
DELETE http://host/service/Categories(1)/Products/$ref?$id=../../Products(0)
DELETE http://host/service/Products(0)/Category/$ref
```