Add filter aggregation doc.

docs/Aggregation/Filter.md

+# Filter Aggregation
+
+> More info about filter aggregation is in the [official elasticsearch docs][1]
+
+Defines a single bucket of all the documents in the current document set context that
+match a specified filter. Often this will be used to narrow down the current aggregation
+context to a specific set of documents.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_red_products" : {
+            "filter" : { "term": { "color": "red" } },
+            "aggs" : {
+                "agg_avg_price" : { "avg" : { "field" : "price" } }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$termFilter = new TermFilter('color', 'red');
+$avgAggregation = new AvgAggregation('avg_price', 'price');
+
+$filterAggregation = new FilterAggregation('grades_stats', $termFilter);
+$filterAggregation->addAggregation($avgAggregation);
+
+$search = new Search();
+$search->addAggregation($filterAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-filter-aggregation.html