Add config file example to variables.md
Add config file example to variables.md

Add a config file example for setting .Site.Params to variables.md. This addresses my confusion in [issue #766](https://github.com/spf13/hugo/issues/766).

tubo028/hugo,anthonyfok/hugo,Zenithar/hugo,ifraixedes/hugo,mdhender/hugo,abourget/hugo,mayah/hugo,fedelibre/hugo,derekperkins/hugo,ryanclarke/hugo,fhergomez/hugo,badoet/hugo,halostatue/hugo,tubo028/hugo,mdhender/hugo,mitchchn/hugo,tatsushid/hugo,gohugoio/hugo,halostatue/hugo,fedelibre/hugo,bogem/hugo,bogem/hugo,bramp/hugo,Zenithar/hugo,soundrussian/hugo,mdhender/hugo,gohugoio/hugo,mitchchn/hugo,bramp/hugo,soundrussian/hugo,ifraixedes/hugo,vbatoufflet/hugo,fedelibre/hugo,digitalcraftsman/hugo,jygastaud/hugo,tischda/hugo,anthonyfok/hugo,MarkDBlackwell/hugo,MarkDBlackwell/hugo,ffrizzo/hugo,coderzh/hugo,melo/hugo,tjyang/hugo,anthonyfok/hugo,vbatoufflet/hugo,ffrizzo/hugo,kujohn/hugo,coderzh/hugo,sprig/hugo,mdhender/hugo,tjyang/hugo,badoet/hugo,sprig/hugo,gohugoio/hugo,jygastaud/hugo,fhergomez/hugo,melo/hugo,fedelibre/hugo,derekperkins/hugo,tatsushid/hugo,ryanclarke/hugo,digitalcraftsman/hugo,tischda/hugo,mayah/hugo,kujohn/hugo,abourget/hugo