Add geo hash grid aggregation doc.

docs/Aggregation/GeoHashGrid.md

+# Geo Hash Grid Aggregation
+
+> More info about geo hash grid aggregation is in the [official elasticsearch docs][1]
+
+A multi-bucket aggregation that works on geo_point fields and groups points into buckets
+that represent cells in a grid.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_GrainGeoHashGrid" : {
+            "geohash_grid" : {
+                "field" : "location",
+                "precision" : 3
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$geoHashGridAggregation = new GeoHashGridAggregation(
+    'GrainGeoHashGrid',
+    'location',
+    3
+);
+
+$search = new Search();
+$search->addAggregation($geoHashGridAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-geohashgrid-aggregation.html