improve osd help by adding example of activation by paths

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #74 from alfredodeza/4954

improve osd help by adding example of activation by paths
update the changelog with all changes

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>