improve documentation about deb_src in readme
improve documentation about deb_src in readme

chef-cookbooks/apt,puckel/apt,BarthV/apt,mchesnut/apt,edmunds-chef/apt,nilsga/apt,whiteley/apt,gswallow/apt,pwnieexpress/apt,portertech/apt,NREL-cookbooks/apt,chrisroberts/apt,Shopify/chef-apt,ortoo/apt,MattMencel/apt,ikkyotech/OpsworksNodeAddons,hw-cookbooks/apt,opscode-cookbooks/apt,tomgl/apt,patcon/apt,cookbooks/oc-apt,Tealium/apt,puckel/apt,ouranos/apt,cookbooks/hw-apt,h4ck3rm1k3/apt,h4ck3rm1k3/apt,PolicyStat/apt,Alukardd/cookbook-apt,daptiv/apt,llaurent/opscode-cookbooks-apt,eteeselink/apt,rockaboxmedia/apt,Altiscale/apt,protec-cookbooks/apt,mrdavidlaing/apt,wrbooth/apt,swistaczek/apt,Rockstar04/apt,scalarae/apt,BarthV/apt,scalarae/apt,Alukardd/cookbook-apt,pwnieexpress/apt,chef-cookbooks/apt,protec-cookbooks/apt,samstokes/apt,brumschlag/apt,CodePen/apt,PolicyStat/apt,josephholsten/opscode-cookbooks-apt,sabcio/apt,Youscribe/apt,Altiscale/apt,Agiley/apt,knuckolls/apt,otype/chef,jtimberman/apt,aerickson/apt,brocksamson/apt,andrewgross/apt,opscode-cookbooks/apt,tmatilai/chef-apt,cookbooks/nd-chef-opscode-apt,patcon/apt,balmbees/apt,VertiPub/apt,paulschwarz/apt,lewg/chef-apt,Rockstar04/apt,mattray/apt,gravitystorm/apt,brumschlag/apt,yola/apt-cookbook,ikkyotech/OpsworksNodeAddons,democracyworks/apt,VertiPub/apt,Banno/apt,Tealium/apt,MarkRoddy/apt,balmbees/apt,rackspace-cookbooks/rackspace_apt,adambrett-forks/chef-apt,trinitronx/apt,edmunds-chef/apt