Zulip Desktop version 5.3.0 only supported adding custom certificates
inside the application. Starting in version 5.4.0, it also supports
reading from the system certificate store; in the next release (likely
5.5.0) the support for the internal store will be removed.
Document the change, and add explicit instructions on how to add
certificates into the system store on each of the operating systems.
Co-authored-by: Manav Mehta <tmanavmehta@gmail.com>