add @dguido to contributors
add @dguido to contributors

NUKnightLab/slask,mmisiewicz/slask,TetraEtc/limbo,ruhee/limbo,shawnsi/limbo,llimllib/limbo,signalnine/alanabot,freshbooks/limbo,serverdensity/sdbot,michaelMinar/limbo,rizaon/limbo,sentinelleader/limbo,kylemsguy/limbo,wmv/slackbot-python,Marclass/limbo,TetraEtc/limbo,palachu/sdbot,dorian1453/limbo,llimllib/limbo