Update roadmap

Author: xudong963

docs/source/specification/roadmap.md

@@ -61,7 +61,7 @@ to provide:
 - Additional constant folding / partial evaluation [#1070](https://github.com/apache/arrow-datafusion/issues/1070)
 - More sophisticated cost based optimizer for join ordering
 - Implement advanced query optimization framework (Tokomak) #440
-
+- Finer optimizations for group by and aggregate functions
 ## Datasources
 
 - Better support for reading data from remote filesystems (e.g. S3) without caching it locally [#907](https://github.com/apache/arrow-datafusion/issues/907) [#1060](https://github.com/apache/arrow-datafusion/issues/1060)