Add nested aggregation example.

docs/Aggregation/index.md

@@ -16,6 +16,67 @@ $queryArray = $search->toArray();
 There are 2 types of aggregation: bucketing and metric. The only difference in using them is that metric bucketing
 aggregations supports nesting while metric aggregations will ignore any set nested aggregations.
 
+## Nesting aggregations
+
+Bucketing aggregation can have anu number nested aggregations and nesting can go to unlimited depth.
+
+Example nested aggregation.
+```JSON
+{
+    "aggregations": {
+        "agg_color": {
+            "terms": {
+                "field": "color"
+            },
+            "aggregations": {
+                "agg_avg_price": {
+                    "avg": {
+                        "field": "price"
+                    }
+                },
+                "agg_brand": {
+                    "terms": {
+                        "field": "brand"
+                    },
+                    "aggregations": {
+                        "agg_avg_price": {
+                            "avg": {
+                                "field": "price"
+                            }
+                        }
+                    }
+                }
+            }
+        },
+        "agg_avg_price": {
+            "avg": {
+                "field": "price"
+            }
+        }
+    }
+}
+```
+
+```php
+$avgPriceAggregation = new AvgAggregation('avg_price');
+$avgPriceAggregation->setField('price');
+
+$brandTermAggregation = new TermsAggregation('brand');
+$brandTermAggregation->setField('brand');
+$brandTermAggregation->addAggregation($avgPriceAggregation);
+
+$colorTermsAggregation = new TermsAggregation('color');
+$colorTermsAggregation->setField('color');
+$colorTermsAggregation->addAggregation($avgPriceAggregation);
+$colorTermsAggregation->addAggregation($brandTermAggregation);
+
+$search = new Search();
+$search->addAggregation($colorTermsAggregation);
+$search->addAggregation($avgPriceAggregation);
+
+$queryArray = $search->toArray();
+```
+
 ## Metric Aggregations
  - [Avg](Avg.md)
  - [Cardinality](Cardinality.md)