Add grunt logo to readme
Add grunt logo to readme

Closes gh-762.

ThiagoGarciaAlves/grunt,yonglehou/grunt,r14r-work/fork_grunt_master,imjerrybao/grunt,felixgrey/grunt,IveWong/grunt,Chhunlong/grunt,thecc4re/gruntjs,youmyyou/grunt,yuhualingfeng/grunt,fscherwi/grunt,r14r/fork_grunt_master,coldrye-collaboration/grunt,paulcbetts/grunt,eelve/grunt,q310550690/grunt,vladikoff/grunt,FeodorFitsner/grunt,JaKXz/grunt,zackexplosion/grunt,goodeggs/grunt,modulexcite/grunt,abraheem/grunt,arosa81/grunt,lkiarest/grunt,paladox/grunt,yosiazwan/grunt,shama/grunt,pandeysoni/grunt,thejameskyle/grunt