Add link to Predicate gem in README.

README.md

@@ -354,6 +354,28 @@ r.unwrap(:a)                                 # shortcut over unwrap([:a])
 r.where(predicate)                           # alias for restrict(predicate)
 ```
 
+## Supported Predicates
+
+Usual operators are supported and map to their SQL equivalent as expected:
+
+```ruby
+Predicate.eq                                 # =
+Predicate.neq                                # <>
+Predicate.lt                                 # <
+Predicate.lte                                # <=
+Predicate.gt                                 # >
+Predicate.gte                                # >=
+Predicate.in                                 # SQL's IN
+Predicate.is_null                            # SQL's IS NULL
+```
+
+See the [Predicate gem](https://github.com/enspirit/predicate) for a more
+complete list.
+
+Note: predicates that implement specific Ruby algorithms or patterns are
+not compiled to SQL (and more generally not delegated to underlying database
+servers).
+
 ## How is this different?
 
 ### ... from similar libraries?