MANIFEST: Update CONTRIBUTING extension
MANIFEST: Update CONTRIBUTING extension

vcavallo/letsencrypt,thanatos/lets-encrypt-preview,stewnorriss/letsencrypt,jsha/letsencrypt,dietsche/letsencrypt,modulexcite/letsencrypt,digideskio/lets-encrypt-preview,deserted/letsencrypt,beermix/letsencrypt,skynet/letsencrypt,piru/letsencrypt,xgin/letsencrypt,stweil/letsencrypt,luorenjin/letsencrypt,sjerdo/letsencrypt,goofwear/letsencrypt,PeterMosmans/letsencrypt,mrb/letsencrypt,kevinlondon/letsencrypt,DavidGarciaCat/letsencrypt,mitnk/letsencrypt,hlieberman/letsencrypt,g1franc/lets-encrypt-preview,martindale/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,ahojjati/letsencrypt,tdfischer/lets-encrypt-preview,hsduk/lets-encrypt-preview,bsmr-misc-forks/letsencrypt,Sveder/letsencrypt,solidgoldbomb/letsencrypt,rugk/letsencrypt,ruo91/letsencrypt,goofwear/letsencrypt,brentdax/letsencrypt,ghyde/letsencrypt,wteiken/letsencrypt,ghyde/letsencrypt,Sveder/letsencrypt,bestwpw/letsencrypt,armersong/letsencrypt,Jadaw1n/letsencrypt,vcavallo/letsencrypt,hlieberman/letsencrypt,TheBoegl/letsencrypt,skynet/letsencrypt,rugk/letsencrypt,tyagi-prashant/letsencrypt,BillKeenan/lets-encrypt-preview,jsha/letsencrypt,solidgoldbomb/letsencrypt,wteiken/letsencrypt,PeterMosmans/letsencrypt,bestwpw/letsencrypt,jtl999/certbot,Hasimir/letsencrypt,BKreisel/letsencrypt,Jadaw1n/letsencrypt,tyagi-prashant/letsencrypt,rutsky/letsencrypt,kuba/letsencrypt,twstrike/le_for_patching,beermix/letsencrypt,Bachmann1234/letsencrypt,riseofthetigers/letsencrypt,armersong/letsencrypt,rlustin/letsencrypt,mrb/letsencrypt,riseofthetigers/letsencrypt,mitnk/letsencrypt,BillKeenan/lets-encrypt-preview,tdfischer/lets-encrypt-preview,piru/letsencrypt,letsencrypt/letsencrypt,diracdeltas/lets-encrypt-preview,diracdeltas/lets-encrypt-preview,twstrike/le_for_patching,TheBoegl/letsencrypt,jmhodges/letsencrypt,fmarier/letsencrypt,rlustin/letsencrypt,sapics/letsencrypt,sjerdo/letsencrypt,lbeltrame/letsencrypt,xgin/letsencrypt,jtl999/certbot,lmcro/letsencrypt,lbeltrame/letsencrypt,VladimirTyrin/letsencrypt,g1franc/lets-encrypt-preview,bsmr-misc-forks/letsencrypt,rutsky/letsencrypt,Jonadabe/letsencrypt,jmhodges/letsencrypt,brentdax/letsencrypt,ahojjati/letsencrypt,DavidGarciaCat/letsencrypt,deserted/letsencrypt,luorenjin/letsencrypt,dietsche/letsencrypt,thanatos/lets-encrypt-preview,Jonadabe/letsencrypt,modulexcite/letsencrypt,jmaurice/letsencrypt,martindale/letsencrypt,ruo91/letsencrypt,sapics/letsencrypt,jmaurice/letsencrypt,fmarier/letsencrypt,BKreisel/letsencrypt,VladimirTyrin/letsencrypt,lmcro/letsencrypt,kuba/letsencrypt,digideskio/lets-encrypt-preview,hsduk/lets-encrypt-preview,Bachmann1234/letsencrypt,kevinlondon/letsencrypt,Hasimir/letsencrypt,stewnorriss/letsencrypt