README.rst: add comment to checkout git submodules (cookbooks)

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #2 from dalgaaf/wip-da-comment

README.rst: add comment to checkout git submodules (cookbooks)

Reviewed-by: Sage Weil <sage@inktank.com>