Add geo bounds aggregation doc.

docs/Aggregation/GeoBounds.md

+# Geo Bounds Aggregation
+
+> More info about geo bounds aggregation is in the [official elasticsearch docs][1]
+
+A metric aggregation that computes the bounding box containing all geo_point values for a field.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_viewport" : {
+            "geo_bounds" : {
+                "field" : "location",
+                "wrap_longitude" : true
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$geoBoundsAggregation = new GeoBoundsAggregation('viewport', 'location');
+
+$search = new Search();
+$search->addAggregation($geoBoundsAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-geobounds-aggregation.html