Add geo distance aggregation doc.

docs/Aggregation/GeoDistance.md

+# Geo Distance Aggregation
+
+> More info about geo distance aggregation is in the [official elasticsearch docs][1]
+
+A multi-bucket aggregation that works on geo_point fields
+and conceptually works very similar to the range aggregation.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "rings_around_amsterdam" : {
+            "geo_distance" : {
+                "field" : "location",
+                "origin" : "52.3760, 4.894",
+                "ranges" : [
+                    { "to" : 100 },
+                    { "from" : 100, "to" : 300 },
+                    { "from" : 300 }
+                ]
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$geoDistanceAggregation = new GeoDistanceAggregation(
+    'rings_around_amsterdam',
+    'location',
+    '52.3760, 4.894',
+    [
+        ['to' => 100],
+        ['from' => 100, 'to' => 300],
+        ['from' => 300],
+    ]
+);
+
+$search = new Search();
+$search->addAggregation($geoDistanceAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-geodistance-aggregation.html