Update Showdown link in Documentation
Update Showdown link in Documentation

Resolves #39

Attacklab.net is no longer available, and this Github repo gives credit to the original authors, and has a test suite, etc.  Looks better than a dead link.
jrvidal/docco,lbn/docco,mhkeller/docco,whitetrefoil/docco,erikdiessel/plidoc,gvithellyna/docco,tbremer/docco,JonathanMatthey/codeconv,lobrien/docco,gvithellyna/docco,lobrien/docco,tbremer/docco,ShawnHuang/docco-multidir,jrvidal/docco,actano/docco,DoomHammer/docco,guigrpa/docco,mhkeller/docco,guigrpa/docco-extended,oponder/docco,Hellyna/docco,oponder/docco,robertmackenzie/docco,lbn/docco,osharim/docco,guigrpa/docco-extended,jalleyne/docco,HubSpot/docco,STRd6/docco,whitetrefoil/docco,MaxwellRebo/docco,jalleyne/docco,erikdiessel/plidoc,guigrpa/docco,ShawnHuang/docco-multidir,osharim/docco,Hellyna/docco,DoomHammer/docco,MaxwellRebo/docco,robertmackenzie/docco