Add section "Bot accounts"
    
    This documents the policy adopted in
    https://pagure.io/fesco/issue/2624.
    
    I added the part about releng, because it is useful as a hint what to
    do in case of problems. I didn't add the part about FESCo, because
    FESCo serves as the general "escalation path" for various issues and I
    don't think we need to mention this specifically here.