[SSSD] [sssd PR#168][synchronized] DOC: Deprecate README, add README.md

[jhrozek]

   URL: https://github.com/SSSD/sssd/pull/168
Author: jhrozek
 Title: #168: DOC: Deprecate README, add README.md
Action: synchronized

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/168/head:pr168
git checkout pr168