Add global aggregation doc.

docs/Aggregation/Global.md

+# Global Aggregation
+
+> More info about cardinality aggregation is in the [official elasticsearch docs][1]
+
+Defines a single bucket of all the documents within the search execution
+context. This context is defined by the indices and the document types
+you’re searching on, but is **not influenced** by the search query itself.
+
+## Simple example
+
+```JSON
+{
+    "aggregations": {
+        "agg_all_products": {
+            "global": {},
+            "aggregations": {
+                "agg_avg_price": {
+                    "avg": {
+                        "field": "price"
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$avgAggregation = new AvgAggregation('avg_price', 'price');
+$globalAggregation = new GlobalAggregation('all_products');
+$globalAggregation->addAggregation($avgAggregation);
+
+$search = new Search();
+$search->addAggregation($globalAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-global-aggregation.html