router.py: Filled in missing docblock.

BUG=None
TEST=None

Change-Id: I0bd43fa4a9e11e183a38109d24d3ba13bc0c6e62
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1670918
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
diff --git a/api/router.py b/api/router.py
index 72a1bbf..a75d4dc 100644
--- a/api/router.py
+++ b/api/router.py
@@ -3,7 +3,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-"""TODO: FILE DOC BLOCK"""
+"""Router class for the Build API.
+
+Handles routing requests to the appropriate controller and handles service
+registration.
+"""
 
 from __future__ import print_function