Fix debian repo info in README

The original example will fail because of "Permission denied" (the scope of sudo does not apply
to the stdout append). This variant will work for non-root users.

Also modified it to the best practice of adding custom repositories to their own files within
/etc/apt/sources.list.d/foobar.list
Merge pull request #21 from colszowka/patch-1

Fix debian repo info in README