Document gotcha with string interpolation
Document gotcha with string interpolation

Resolves #832

stanhu/haml,haml/haml,tangposmarvin/haml,macressler/haml,nazar-pc/haml,macressler/haml,keithpitty/haml,stanhu/haml,nazar-pc/haml,tangposmarvin/haml,haml/haml,dg1dd1ngs/haml,keithpitty/haml,haml/haml,dg1dd1ngs/haml,haml/haml