add Debian build steps in INSTALL
diff --git a/INSTALL b/INSTALL
index 251561c..0e4cec8 100644
--- a/INSTALL
+++ b/INSTALL
@@ -8,3 +8,8 @@
 Cleaning is the usual:
 
   make clean
+
+To make a Debian package:
+
+  git checkout debian-master
+  make deb