README: add protocol to URL so it isn't a relative link

Change-Id: I1535a1b58ec0738741f84c033caa28e46c598da7
diff --git a/README.md b/README.md
index bc85256..8c6b340 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 Hafnium is a hypervisor, initially supporting aarch64 (64-bit ARMv8 CPUs).
 
-Further details are available in the [design doc](go/hafnium-dd).
+Further details are available in the [design doc](https://goto.google.com/hafnium-dd).
 
 ## Getting the source code