Update doc/articles/handling-private-packages-with-satis.md

Added the stability flag to this article, was a little confusing as to why satis didn't install correctly using this article. Turned out the github composer/satis README had the answer so i've put it here also.

Matt
Merge pull request #1512 from mattkirwan/patch-1

Added stability flag to the install code: doc/articles/handling-private-packages-with-satis.md