Added comments about search/index with cyrus 3

Summary:
On debian10 systems with cyrus 3 the squatter command doesn't accept the
-s argument anymore and squatter will not run if specified. Also the new
parameter search_engine is introduced. Without search_engine set the squat files
will not be used.

This diff just adds some comments to features not enabled by default.

Reviewers: PyKolab Developers, vanmeeuwen

Reviewed By: PyKolab Developers, vanmeeuwen

Differential Revision: https://git.kolab.org/D919