add contributions section to README
add contributions section to README

zbintliff/sensu-plugin,nyxcharon/sensu-plugins-fleet,kelcecil/sensu-plugins-bitbucket,nyxcharon/sensu-plugins-vault,sensu-plugins/sensu-plugin,nyxcharon/sensu-plugins-last-modified,nyxcharon/sensu-plugins-bitbucket,upfluence/sensu-plugin,somic/sensu-plugin,nyxcharon/sensu-plugins-quay