Add the gst-adder.sh example

diff --git a/shell/gst-adder.sh b/shell/gst-adder.sh
new file mode 100755 (executable)
index 0000000..3cf2a2a
--- /dev/null
+++ b/shell/gst-adder.sh
@@ -0,0 +1,5 @@
+#!/bin/sh
+
+gst-launch-1.0 adder name=mix ! audioconvert ! autoaudiosink \
+  audiotestsrc wave=2 ! mix. \
+  audiotestsrc wave=5 ! mix.