Add terms aggregation doc.

docs/Aggregation/Terms.md

+# Terms Aggregation
+
+> More info about terms aggregation is in the [official elasticsearch docs][1]
+
+A multi-bucket value source based aggregation where buckets are dynamically
+built - one per unique value.
+
+## Simple example
+
+```JSON
+{
+    "aggregations" : {
+        "agg_genders" : {
+            "terms" : { "field" : "gender" }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$termsAggregation = new TermsAggregation('genders', 'gender');
+
+$search = new Search();
+$search->addAggregation($termsAggregation);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-terms-aggregation.html