add notice for users to switch their checkout to main

When users run `repo sync` and still using the master branch,
emit a notice for them to switch to the main branch.

BUG=chromium:1177742
TEST=`repo sync` emits this message

Change-Id: I6befd38d570efc05c6c5dde7961b78f9641257d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/manifest/+/2779715
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/full.xml b/full.xml
index bc25fba..b6bb715 100644
--- a/full.xml
+++ b/full.xml
@@ -1,5 +1,16 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <manifest>
+  <notice>
+  ********************************************************************************
+  This repo has migrated to the "main" branch.  Please update your checkout:
+  $ repo init -b main
+  $ repo sync
+
+  For more details, see:
+  https://docs.google.com/document/d/1isVGvU33xNC_yIFp8dLl3Kn6WtGj3g7lBceuq91-duU/preview
+  ********************************************************************************
+  </notice>
+
   <include name="_remotes.xml" />
   <default revision="refs/heads/main"
            remote="cros"