added documentation for the query

docs/Query/SpanContaining.md

+# Span Containing query
+
+> More info about Boosting query is in the [official elasticsearch docs][1]
+
+Returns matches which enclose another span query.
+
+```JSON
+{
+    "span_containing" : {
+        "little" : {
+            "span_term" : { "field1" : "foo" }
+        },
+        "big" : {
+            "span_near" : {
+                "clauses" : [
+                    { "span_term" : { "field1" : "bar" } },
+                    { "span_term" : { "field1" : "baz" } }
+                ],
+                "slop" : 5,
+                "in_order" : true
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$spanTermQuery = new SpanTermQuery('field1', 'foo');
+$spanNearQuery = new SpanNearQuery();
+
+$spanNearQuery->setSlop(5);
+$spanNearQuery->addParameter('in_order', true);
+$spanNearQuery->addQuery(new SpanTermQuery('field1', 'bar'));
+$spanNearQuery->addQuery(new SpanTermQuery('field1', 'baz'));
+
+$spanContainingQuery = new SpanContainingQuery(
+    $spanTermQuery,
+    $spanNearQuery
+);
+```
+
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-span-containing-query.html