Add geo polygon filter doc.

docs/Filter/GeoPolygon.md

+# Geo Polygon Filter
+
+> More info about geo polygon range filter is in the [official elasticsearch docs][1]
+
+A filter allowing to include hits that only fall within a polygon of points.
+
+## Simple example
+
+```JSON
+{
+    "filtered" : {
+        "query" : {
+            "match_all" : {}
+        },
+        "filter" : {
+            "geo_polygon" : {
+                "person.location" : {
+                    "points" : [
+                        {"lat" : 40, "lon" : -70},
+                        {"lat" : 30, "lon" : -80},
+                        {"lat" : 20, "lon" : -90}
+                    ]
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$geoPolygonFilter = new GeoPolygonFilter(
+    'person.location',
+    [
+        ['lat' => 40, 'lon' => -70],
+        ['lat' => 30, 'lon' => -80],
+        ['lat' => 20, 'lon' => -90],
+    ]
+);
+
+$search = new Search();
+$search->addFilter($geoPolygonFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-geo-polygon-filter.html