Add iter_combinations to examples' README.md (#2266)

Added a short description of the example.

Fixes #2263.

Author: Nilirad

examples/README.md

@@ -160,6 +160,7 @@ Example | File | Description
 `event` | [`ecs/event.rs`](./ecs/event.rs) | Illustrates event creation, activation, and reception
 `fixed_timestep` | [`ecs/fixed_timestep.rs`](./ecs/fixed_timestep.rs) | Shows how to create systems that run every fixed timestep, rather than every tick
 `hierarchy` | [`ecs/hierarchy.rs`](./ecs/hierarchy.rs) | Creates a hierarchy of parents and children entities
+`iter_combinations` | [`ecs/iter_combinations.rs`](./ecs/iter_combinations.rs) | Shows how to iterate over combinations of query results.
 `parallel_query` | [`ecs/parallel_query.rs`](./ecs/parallel_query.rs) | Illustrates parallel queries with `ParallelIterator`
 `query_bundle` | [`ecs/query_bundle.rs`](./ecs/query_bundle.rs) | Shows how to query entities that contain components in a `Bundle`
 `removal_detection` | [`ecs/removal_detection.rs`](./ecs/removal_detection.rs) | Query for entities that had a specific component removed in a previous stage during the current frame.