added documentation

docs/Aggregation/SamplerAgg.md

+# Sampler Aggregation
+
+> More info about histogram aggregation is in the [official elasticsearch docs][1]
+
+A filtering aggregation used to limit any sub aggregations' processing to a sample of the top-scoring documents. Optionally,
+diversity settings can be used to limit the number of matches that share a common value such as an "author".
+
+## Simple example
+
+```JSON
+{
+    "aggregations": {
+        "sample": {
+            "sampler": {
+                "shard_size": 200,
+                "field" : "user.id"
+            },
+            "aggs": {
+                 "keywords": {
+                     "significant_terms": {
+                         "field": "text"
+                     }
+                 }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$samplerAggregation = new SamplerAggregation('sample', 'user.id', 200);
+$samplerAggregation->addAggregation(
+    new SignificantTermsAggregation('keywords', 'text')
+);
+
+$search = new Search();
+$search->addAggregation($samplerAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-sampler-aggregation.html