add security mail to contributing.md
add security mail to contributing.md

see exist-db/exist#1554
windauer/exist,eXist-db/exist,hungerburg/exist,adamretter/exist,eXist-db/exist,ambs/exist,adamretter/exist,windauer/exist,windauer/exist,ljo/exist,eXist-db/exist,eXist-db/exist,dizzzz/exist,hungerburg/exist,opax/exist,wolfgangmm/exist,adamretter/exist,wolfgangmm/exist,windauer/exist,lcahlander/exist,lcahlander/exist,adamretter/exist,hungerburg/exist,lcahlander/exist,RemiKoutcherawy/exist,olvidalo/exist,lcahlander/exist,RemiKoutcherawy/exist,wolfgangmm/exist,opax/exist,RemiKoutcherawy/exist,wolfgangmm/exist,lcahlander/exist,olvidalo/exist,ambs/exist,hungerburg/exist,dizzzz/exist,RemiKoutcherawy/exist,ambs/exist,dizzzz/exist,opax/exist,dizzzz/exist,RemiKoutcherawy/exist,wolfgangmm/exist,ljo/exist,opax/exist,olvidalo/exist,ambs/exist,wolfgangmm/exist,eXist-db/exist,dizzzz/exist,opax/exist,adamretter/exist,windauer/exist,adamretter/exist,olvidalo/exist,ljo/exist,windauer/exist,ljo/exist,hungerburg/exist,olvidalo/exist,eXist-db/exist,dizzzz/exist,ambs/exist,ljo/exist,ljo/exist,ambs/exist,RemiKoutcherawy/exist,lcahlander/exist