Add geohash cell filter doc.

docs/Filter/GeohashCell.md

+# Geohash Cell Filter
+
+> More info about geohash cell filter is in the [official elasticsearch docs][1]
+
+The geohash cell filter provides access to a hierarchy of geohashes.
+By defining a geohash cell, only geopoints within this cell will match this filter.
+
+## Simple example
+
+```JSON
+{
+    "filtered" : {
+        "query" : {
+            "match_all" : {}
+        },
+        "filter" : {
+            "geohash_cell": {
+                "pin": {
+                    "lat": 13.4080,
+                    "lon": 52.5186
+                },
+                "precision": 3,
+                "neighbors": true
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$geohashCellFilter = new GeohashCellFilter(
+    'pin',
+    [
+        'lat' => 13.4080,
+        'lon' => 52.5186,
+    ],
+    [
+        'precision' => 3,
+        'neighbors' => true,
+    ]
+);
+
+$search = new Search();
+$search->addFilter($geohashCellFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-geohash-cell-filter.html