Add link to dagger user guide

Test: gitiles
Change-Id: I9c2ab16bf049ee1398d5761e06aa9c6f556d9722
diff --git a/packages/SystemUI/docs/dagger.md b/packages/SystemUI/docs/dagger.md
index 7751bb1..cca2d13 100644
--- a/packages/SystemUI/docs/dagger.md
+++ b/packages/SystemUI/docs/dagger.md
@@ -6,6 +6,8 @@
 
 Go read about Dagger 2.
 
+ - [User's guide](https://google.github.io/dagger/users-guide)
+
 TODO: Add some links.
 
 ## State of the world