Add note about IS_EXTERNAL

b/117345558

Change-Id: I87664c1e5a8ac963b1ab771cc9de0c716f2fd9e2

GettingStarted.md

diff --git a/GettingStarted.md b/GettingStarted.md
index 774a027..b7ffae5 100644
--- a/GettingStarted.md
+++ b/GettingStarted.md
@@ -169,6 +169,12 @@
 m
 ```
 
+If you're not on a Google workstation, or are but do not wish to depend on any cached artifacts,
+set the IS_EXTERNAL flag:
+```
+IS_EXTERNAL=true m
+```
+
 The first time you'll be asked to set a pbuilder mirror site. Use
 http://deb.debian.org/debian.