docs: add sberyozkin as a contributor for code, maintenance

[allcontributors[bot]]

Add @sberyozkin as a contributor for code, maintenance.

This was requested by gastaldi in this comment

[sberyozkin]

Hey @gastaldi Can you please get this bot add Stuart as well ? Or is it all automatic

[gastaldi]

@all-contributors add @stuartwdouglas for code and maintenance

[allcontributors[bot]]

@gastaldi

I've put up a pull request to add @stuartwdouglas! :tada:

[gastaldi]

@sberyozkin added. You just need to specify a comment like https://github.com/quarkiverse/quarkus-kerberos/pull/19#issuecomment-937835609 to have someone added to the list 😉

[sberyozkin]

@gastaldi cool. thanks; will keep in mind as well, cheers