Documentation for query data sources

docs/content/DataSource.md

@@ -0,0 +1,25 @@
+---
+layout: doc_page
+---
+A data source is the Druid equivalent of a database table. However, a query can also masquerade as a data source, providing subquery-like functionality. Query data sources are currently only supported by [GroupBy](GroupByQuery.html) queries.
+
+### Table Data Source
+The table data source the most common type. It's represented by a string, or by the full structure:
+
+```json
+{
+	"type": "table",
+	"name": <string_value>
+}
+```
+
+### Query Data Source
+```json
+{
+	"type": "query",
+	"query": {
+		"type": "groupBy",
+		...
+	}
+}
+```

docs/content/GroupByQuery.md

@@ -47,7 +47,7 @@ There are 9 main parts to a groupBy query:
 |property|description|required?|
 |--------|-----------|---------|
 |queryType|This String should always be "groupBy"; this is the first thing Druid looks at to figure out how to interpret the query|yes|
-|dataSource|A String defining the data source to query, very similar to a table in a relational database|yes|
+|dataSource|A String defining the data source to query, very similar to a table in a relational database, or a [DataSource](DataSource.html) structure.|yes|
 |dimensions|A JSON list of dimensions to do the groupBy over|yes|
 |orderBy|See [OrderBy](OrderBy.html).|no|
 |having|See [Having](Having.html).|no|