Update NOTICE.txt (#957)
Update NOTICE.txt (#957)

(this is the automated NOTICE.txt update for April 2019)
yuya-oc/electron-mattermost,mattermost/desktop,mattermost/desktop,mattermost/desktop,yuya-oc/electron-mattermost,mattermost/desktop,mattermost/desktop,yuya-oc/electron-mattermost