add emacs shell-script tagging

for files that don't start with a #! or end in .sh, the added tags
are nice for emacs users to automatically switch to the right mode.

Change-Id: If4b93e106191bc744ccad8420cef20e751cdf902
diff --git a/stackrc b/stackrc
index f35cdc0..5a4c580 100644
--- a/stackrc
+++ b/stackrc
@@ -221,3 +221,7 @@
 
 # Compatibility until it's eradicated from CI
 USE_SCREEN=${SCREEN_DEV:-$USE_SCREEN}
+
+# Local variables:
+# mode: shell-script
+# End: