Add a reference section about - syntax.
Add a reference section about - syntax.

Closes #1761

askl56/sass,tangposmarvin/sass,ridixcr/sass,danielfein/sass,tangposmarvin/sass,Nzaga/sass,amitsuroliya/sass,jaroot32/sass,nicklima/sass,antonifs/sass,antonifs/sass,hcatlin/sass,jaroot32/sass,nicklima/sass,hcatlin/sass,Galactix/sass,liquidmetal/sass,PMArtz92/sass,rammy7219/sass,Ahnita/sass,danielfein/sass,schneems/sass,xzyfer/sass,schneems/sass,xzyfer/sass,zaru/sass,modulexcite/sass,saper/sass,cmpereirasi/sass,Andrey-Pavlov/sass,Iamronan/sass,rammy7219/sass,nickg33/sass,zaru/sass,amitsuroliya/sass,dylannnn/sass,HugoRLopes/sass,dylannnn/sass,JuanitoFatas/sass,Industrys/sass,OrenBochman/sass,Andrey-Pavlov/sass,StephanieMak/sass,stewx/sass,cmpereirasi/sass,JuanitoFatas/sass,askl56/sass,liquidmetal/sass,srawlins/sass,OrenBochman/sass,nickg33/sass,MetaAshley/sass,saper/sass,PMArtz92/sass,MetaAshley/sass,stewx/sass,modulexcite/sass,srawlins/sass,Nzaga/sass,Ahnita/sass,DanteKaris/sass,ridixcr/sass,lmetla/sass,Anshdesire/sass,Anshdesire/sass,DanteKaris/sass,HugoRLopes/sass,StephanieMak/sass,lmetla/sass,Iamronan/sass,Galactix/sass,Industrys/sass