Added documentation for new virtualenv feature.
Added documentation for new virtualenv feature.

James-Firth/pip,haridsv/pip,fiber-space/pip,wkeyword/pip,jythontools/pip,jamezpolley/pip,ChristopherHogan/pip,techtonik/pip,harrisonfeng/pip,luzfcb/pip,Carreau/pip,zorosteven/pip,davidovich/pip,mindw/pip,ncoghlan/pip,zvezdan/pip,esc/pip,RonnyPfannschmidt/pip,mattrobenolt/pip,pfmoore/pip,blarghmatey/pip,cjerdonek/pip,jmagnusson/pip,h4ck3rm1k3/pip,pjdelport/pip,rbtcollins/pip,techtonik/pip,habnabit/pip,ChristopherHogan/pip,chaoallsome/pip,pypa/pip,KarelJakubec/pip,haridsv/pip,dstufft/pip,graingert/pip,jasonkying/pip,jasonkying/pip,wkeyword/pip,graingert/pip,Gabriel439/pip,dstufft/pip,nthall/pip,benesch/pip,willingc/pip,supriyantomaftuh/pip,Gabriel439/pip,davidovich/pip,jasonkying/pip,mindw/pip,RonnyPfannschmidt/pip,squidsoup/pip,alquerci/pip,mujiansu/pip,alquerci/pip,msabramo/pip,h4ck3rm1k3/pip,benesch/pip,ianw/pip,sigmavirus24/pip,fiber-space/pip,erikrose/pip,KarelJakubec/pip,zorosteven/pip,tdsmith/pip,pradyunsg/pip,caosmo/pip,willingc/pip,pfmoore/pip,ncoghlan/pip,radiosilence/pip,zvezdan/pip,alex/pip,blarghmatey/pip,Ivoz/pip,jmagnusson/pip,xavfernandez/pip,prasaianooz/pip,haridsv/pip,mindw/pip,caosmo/pip,nthall/pip,natefoo/pip,pradyunsg/pip,esc/pip,ianw/pip,zvezdan/pip,supriyantomaftuh/pip,natefoo/pip,prasaianooz/pip,blarghmatey/pip,squidsoup/pip,harrisonfeng/pip,atdaemon/pip,willingc/pip,jmagnusson/pip,tdsmith/pip,harrisonfeng/pip,pjdelport/pip,alex/pip,patricklaw/pip,tdsmith/pip,rouge8/pip,luzfcb/pip,Carreau/pip,qbdsoft/pip,supriyantomaftuh/pip,qbdsoft/pip,natefoo/pip,jamezpolley/pip,qwcode/pip,h4ck3rm1k3/pip,James-Firth/pip,sbidoul/pip,minrk/pip,zenlambda/pip,alex/pip,wkeyword/pip,pypa/pip,erikrose/pip,chaoallsome/pip,yati-sagade/pip,xavfernandez/pip,minrk/pip,yati-sagade/pip,esc/pip,sigmavirus24/pip,techtonik/pip,luzfcb/pip,qwcode/pip,sigmavirus24/pip,zorosteven/pip,jythontools/pip,dstufft/pip,erikrose/pip,cjerdonek/pip,atdaemon/pip,KarelJakubec/pip,yati-sagade/pip,pjdelport/pip,mujiansu/pip,qbdsoft/pip,ncoghlan/pip,caosmo/pip,jythontools/pip,zenlambda/pip,msabramo/pip,domenkozar/pip,RonnyPfannschmidt/pip,zenlambda/pip,ChristopherHogan/pip,atdaemon/pip,benesch/pip,habnabit/pip,sbidoul/pip,mattrobenolt/pip,davidovich/pip,Ivoz/pip,rbtcollins/pip,squidsoup/pip,James-Firth/pip,habnabit/pip,prasaianooz/pip,chaoallsome/pip,nthall/pip,xavfernandez/pip,fiber-space/pip,graingert/pip,jamezpolley/pip,mujiansu/pip,Gabriel439/pip,rouge8/pip,rbtcollins/pip,patricklaw/pip,rouge8/pip