Add geo distance filter doc.

docs/Filter/GeoDistance.md

+# Geo Distance Filter
+
+> More info about geo distance filter is in the [official elasticsearch docs][1]
+
+Filters documents that include only hits that exists within a specific distance from a geo point.
+
+## Simple example
+
+```JSON
+{
+    "filtered" : {
+        "query" : {
+            "match_all" : {}
+        },
+        "filter" : {
+            "geo_distance" : {
+                "distance" : "200km",
+                "pin.location" : {
+                    "lat" : 40,
+                    "lon" : -70
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$geoDistanceFilter = new GeoDistanceFilter(
+    'pin.location',
+    '200km',
+    ['lat' => 40, 'lon' => -70]
+);
+
+$search = new Search();
+$search->addFilter($geoDistanceFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-geo-distance-filter.html