Update TODO.rst to point the StoryBoard

This commit updates TODO.rst to point the StoryBoard. Because it's hard
to keep up-to-date. I already dumped out original work items to the
StoryBoard[1] with "todo" tag.

[1] https://storyboard.openstack.org/#!/project/747

Change-Id: I96885ebcc6d2235ffa7b1245759691f04c0965a5

TODO.rst

@@ -1,14 +1,4 @@
 Work Items for Subunit2SQL
 ==========================
 
- * Add attachments support to the sql2subunit utility
- * Enable sqlite support for the data migrations to enable using sqlite as
-   a database backend
- * Add more unit tests
-   * DB API unit tests
-   * write_subunit module
- * Flesh out query side of DB API to make it useful for building additional
-   tooling.
- * Investigate dropping oslo.db from requirements to enable using other
-   config/cli tooling
- * Add a subunit2sql repository type to the testrepository project
+You can see work items on the `StoryBoard<https://storyboard.openstack.org/#!/project/747`_.