go.tools/dashboard/watcher: commit watcher rewrite

LGTM=bradfitz
R=bradfitz, alex.brainman, dvyukov
CC=golang-codereviews
https://golang.org/cl/155730043
diff --git a/README b/README
index 2d79ab7..1224450 100644
--- a/README
+++ b/README
@@ -10,6 +10,8 @@
           builds (using the builder in single-shot mode) in Docker containers.
 env/:     configuration files describing the environment of builders.
           Many builders are still configured ad-hoc.
+watcher/: a daemon that watches for new commits to the Go repository and
+          its sub-repositories, and notifies the dashboard of those commits.
 
 If you wish to run a Go builder, please email golang-dev@googlegroups.com