Update README for focusMainFrame.
Update README for focusMainFrame.

tuchangwei/vimium,mrmr1993/vimium,Aleroniponi/vimium,decaffeinate-examples/vimium,mrmr1993/vimium,smblott-github/vimium,wildeyes/vimium,justalittlenoob/vimium,willsALMANJ/vimium,PickRelated/vimium,dimatter/vimium,PickRelated/vimium,decaffeinate-examples/vimium,zhangmin510/vimium,borgified/vimium,poacher2k/vimium,Lw-Cui/vimium,wildeyes/vimium,wildeyes/vimium,hhsue/vimium,bkudria/vimium,justalittlenoob/vimium,VPashkov/vimium,elderbas/vimium,Aleroniponi/vimium,elegantwww/vimium,Robinson10240/vimium,csmalin/vimium,PrestanceDesign/vimium,douglas-larocca/vimium,douglas-larocca/vimium,decaffeinate-examples/vimium,zhangmin510/vimium,poacher2k/vimium,Aleroniponi/vimium,Lw-Cui/vimium,elderbas/vimium,smblott-github/vimium,philc/vimium,willsALMANJ/vimium,tuchangwei/vimium,Robinson10240/vimium,elegantwww/vimium,philc/vimium,PrestanceDesign/vimium,bkudria/vimium,PickRelated/vimium,VPashkov/vimium,mrmr1993/vimium,gdh1995/vimium,gdh1995/vimium,poacher2k/vimium,dimatter/vimium,PrestanceDesign/vimium,VPashkov/vimium,cyrixhero/vimium,elegantwww/vimium,csmalin/vimium,borgified/vimium,Lw-Cui/vimium,elderbas/vimium,douglas-larocca/vimium,bkudria/vimium,willsALMANJ/vimium,dimatter/vimium,smblott-github/vimium,justalittlenoob/vimium,philc/vimium,hhsue/vimium,hhsue/vimium,cyrixhero/vimium,borgified/vimium,gdh1995/vimium,Robinson10240/vimium,cyrixhero/vimium,tuchangwei/vimium,zhangmin510/vimium,csmalin/vimium