Add aggregation doc.

docs/Aggregation/index.md

+# Aggregation
+
+Objective aggregation builder represents all available [Elasticsearch aggregations][1].
+
+
+To form an aggregation you have to create `Search` object. See below an example of min aggregation usage.
+
+```php
+$search = new Search();
+$minAggregation = new MinAggregation('min_agg');
+$minAggregation->setField('price');
+$search->addAggregation($minAggregation);
+$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.
+
+## Metric Aggregations
+ - [Avg](Avg.md)
+ - [Cardinality](Cardinality.md)
+ - [ExtendedStats](ExtendedStats.md)
+ - [Max](Max.md)
+ - [Min](Min.md)
+ - [PercentileRanks](PercentileRanks.md)
+ - [Percentiles](Percentiles.md)
+ - [Stats](Stats.md)
+ - [Sum](Sum.md)
+ - [TopHits](TopHits.md)
+ - [ValueCount](ValueCount.md)
+ 
+## Bucketing Aggregations
+ - [Children](Children.md)
+ - [DateRange](DateRange.md)
+ - [Filter](Filter.md)
+ - [Filters](Filters.md)
+ - [GeoBounds](GeoBounds.md)
+ - [GeoDistance](GeoDistance.md)
+ - [GeoHashGrid](GeoHashGrid.md)
+ - [Global](Global.md)
+ - [Histogram](Histogram.md)
+ - [Ipv4Range](Ipv4Range.md)
+ - [Missing](Missing.md)
+ - [Nested](Nested.md)
+ - [Range](Range.md)
+ - [ReverseNested](ReverseNested.md)
+ - [Terms](Terms.md)
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations.html

docs/index.md

@@ -7,7 +7,7 @@ Everything starts from the `Search` object. We recommend first to take a look at
 ### Topics:
 - [Build Queries](Query/index.md)
 - [Build Filters](Filter/index.md)
-- [Build Aggregations]
+- [Build Aggregations](Aggregation/index.md)
 
 ### How to
 - [How to combine `Filter` and `Query`]