add info about debian package to README

Author: zilder

README.md

@@ -3,5 +3,6 @@
 This repo contains everything you might need to create pg_pathman's system package. Currently we provide build scripts for:
 
 * [Archlinux](https://www.archlinux.org/) (master branch)
+* [Debian](https://www.debian.org/)/[Ubuntu](http://www.ubuntu.com/) (master branch, x86-64)
 
 One day this list will definitely become larger.