Add missing aggregation doc.

docs/Aggregation/Missing.md

+# Missing Aggregation
+
+> More info about missing aggregation is in the [official elasticsearch docs][1]
+
+A field data based single bucket aggregation, that creates a bucket of all documents
+in the current document set context that are missing a field value.
+
+## Simple example
+
+```JSON
+{
+     "aggregations" : {
+         "agg_products_without_a_price" : {
+             "missing" : { "field" : "price" }
+         }
+     }
+ }
+```
+
+And now the query via DSL:
+
+```php
+$missingAggregation = new MissingAggregation('products_without_a_price', 'price');
+
+$search = new Search();
+$search->addAggregation($missingAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-missing-aggregation.html