add note about librbd1 and librados on install docs

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
when purging/uninstalling mention note about librados and librbd

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #182 from alfredodeza/7617

alert about some packages that will not get removed