Added a list of third-party libraries.
Added a list of third-party libraries.

Closes #417.

jasw/metrics,fcrepo4-archive/metrics,slovdahl/metrics,Banno/metrics,mnuessler/metrics,thelastpickle/metrics,mtakaki/metrics,wfxiang08/metrics,randomstatistic/metrics,slachiewicz/metrics,cirrus-dev/metrics,fcrepo4-archive/metrics,egymgmbh/metrics,mattnelson/metrics,jplock/metrics,gburton1/metrics,dropwizard/metrics,kevintvh/metrics,mveitas/metrics,ohr/metrics,dropwizard/metrics,valery1707/dropwizard-metrics,timezra/metrics,AltitudeDigital/metrics,rexren/metrics,dropwizard/metrics,unitsofmeasurement/metrics,bentatham/metrics,gburton1/metrics,mspiegel/metrics,ind9/metrics,infusionsoft/yammer-metrics,signalfx/metrics,ChetnaChaudhari/metrics,mveitas/metrics,chenxianghua2014/metrics,mt0803/metrics,tempredirect/metrics,infusionsoft/yammer-metrics