added boosting query docs

docs/Query/Boosting.md

+# Boosting query
+
+> More info about Boosting query is in the [official elasticsearch docs][1]
+
+Lets take an example to write a query with Elasticsearch DSL.
+
+```JSON
+{
+    "boosting" : {
+        "positive" : {
+            "term" : {
+                "field1" : "value1"
+            }
+        },
+        "negative" : {
+            "term" : {
+                "field2" : "value2"
+            }
+        },
+        "negative_boost" : 0.2
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$termQuery1 = new TermQuery("field1", "value1");
+$termQuery2 = new TermQuery("field2", "value2");
+
+$boostingQuery = new BoostingQuery($termQuery1, $termQuery2, 0.2);
+
+$search = new Search();
+$search->addQuery($boostingQuery);
+
+$queryArray = $search->toArray();
+```
+
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-boosting-query.html