Mention DCO requirement in CONTRIBUTING.md
Mention DCO requirement in CONTRIBUTING.md

Address one of the silver CII Best Practices.

Signed-off-by: thc202 <43af82ef30812a20fbcf38b99a6f72cdd3a89ac1@gmail.com>

meitar/zaproxy,thc202/zaproxy,psiinon/zaproxy,kingthorin/zaproxy,Ali-Razmjoo/zaproxy,kingthorin/zaproxy,Ali-Razmjoo/zaproxy,gmaran23/zaproxy,gmaran23/zaproxy,meitar/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,thc202/zaproxy,psiinon/zaproxy,thc202/zaproxy,thc202/zaproxy,Ali-Razmjoo/zaproxy,zaproxy/zaproxy,zaproxy/zaproxy,thc202/zaproxy,gmaran23/zaproxy,zaproxy/zaproxy,meitar/zaproxy,zaproxy/zaproxy,Ali-Razmjoo/zaproxy,Ali-Razmjoo/zaproxy,kingthorin/zaproxy,meitar/zaproxy,Ali-Razmjoo/zaproxy,gmaran23/zaproxy,zaproxy/zaproxy,Ali-Razmjoo/zaproxy,kingthorin/zaproxy,psiinon/zaproxy,thc202/zaproxy,meitar/zaproxy,psiinon/zaproxy,meitar/zaproxy,meitar/zaproxy,kingthorin/zaproxy,gmaran23/zaproxy,psiinon/zaproxy,thc202/zaproxy,gmaran23/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,meitar/zaproxy,zaproxy/zaproxy,zaproxy/zaproxy,psiinon/zaproxy