Add link to hologram-github-theme in README.md
Add link to hologram-github-theme in README.md

donnoman/hologram,sbrady/hologram,angelabier1/hologram,trulia/hologram,electronspin/hologram,askl56/hologram,askl56/hologram,angelabier1/hologram,electronspin/hologram,trulia/hologram,landsurveyorsunited/hologram,sbrady/hologram,mike6398/hologram,donnoman/hologram,hunj/hologram,hunj/hologram,mike6398/hologram,landsurveyorsunited/hologram