Please document a working regexp example

[cweiske]

The issue from rspamd/rspamd.com#423 is still valid:

The regexp module documentation does not contain a working example.

Please show an example (and document it) that can go in local.d/regexp.conf - Ideally one that will immediately a) learn spam and reject or b) drop or discard

It would also help if using a regexp on a specific header like "subject" is be shown.