Add nested filter doc.

docs/Filter/Nested.md

+# Nested Filter
+
+> More info about nested filter is in the [official elasticsearch docs][1]
+
+Nested filter allows to filter nested objects / documents (see nested mapping).
+The filter is executed against the nested objects / documents as if they
+were indexed as separate documents (they are, internally) and resulting
+in the root parent doc (or parent nested mapping).
+
+## Simple example
+
+```JSON
+{
+    "filtered" : {
+        "query" : { "match_all" : {} },
+        "filter" : {
+            "nested" : {
+                "path" : "obj1",
+                "filter" : {
+                    "bool" : {
+                        "must" : [
+                            {
+                                "term" : {"obj1.name" : "blue"}
+                            },
+                            {
+                                "range" : {"obj1.count" : {"gt" : 5}}
+                            }
+                        ]
+                    }
+                },
+                "_cache" : true
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$termFilter = new TermFilter('obj1.name', 'blue');
+$rangeFilter = new RangeFilter('obj1.count', ['gt' => 5]);
+
+$boolFilter = new BoolFilter();
+$boolFilter->add($termFilter);
+$boolFilter->add($rangeFilter);
+
+$nestedFilter = new NestedFilter('obj1', $boolFilter, ['_cache' => true]);
+
+$search = new Search();
+$search->addFilter($nestedFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-nested-filter.html