Add security policy

[solth]

Add security policy with information about supported versions of Kitodo.Production.

As discussed with @kitodo/kitodo-board only the version which is currently in development - 3.7 right now - will receive active development while the last release (3.6 at the moment of this writing) will get security fixes.

[stefanCCS]

Content (=support for security patches for current dev version and last published Release) is ok.
Questions I have:

• Is this email "[email protected]" existing? Who gets these emails?
• As I could not see (or at least do not know how) the interpreted view of the MD-File, I have copied the content to an external viewer (NotePad++), and there the heavy_check_mark and heavy_multiplication_x were not interpreted - see
  

[solth]

@stefanCCS thanks for the review. The e-mail address (and the whole template) is more or less the same as in Kitodo.Presentation (just a little simplified because of one less column), see https://github.com/kitodo/kitodo-presentation/blob/master/SECURITY.md and pretty much the standard template Github suggests when creating a new security policy from scratch.

I neither know whether the e-mail address actually exists nor who reads it but I assumed it would be the appropriate address since it's already in use for presentation. Perhaps @sebastian-meyer or @kselmi can answer this question?

The check marks (and "multiplication x") are not displayed locally because they are Github emojis that only work on this platform (ctrl +f for :heavy_check_mark: in this cheatsheet: https://github.com/ikatyang/emoji-cheat-sheet/blob/master/README.md)