Add note on bazel apt source for external users

Change-Id: If22cc97e2aea8083e71dd9d1d705d714f51e91f7

GettingStarted.md

diff --git a/GettingStarted.md b/GettingStarted.md
index 0ac2ac0..537e9a1 100644
--- a/GettingStarted.md
+++ b/GettingStarted.md
@@ -52,6 +52,9 @@
 source build/setup.sh
 ```
 
+For non-gLinux systems: Add the bazel apt repository to your machines sources.
+Instructions @ https://bazel.build/install/ubuntu#add-dis-uri
+
 Install the prerequisites:
 
 ```bash