Documenting the path option

When using Bourbon with e.g. Middleman or Jekyll I find myself always moving the installed files after installation manually. So I took a look at the source and discovered that I can specify a destination path as well when running the install task.

Since the target “audience” might contain many front–end devs and maybe even designers who aren’t (very) familiar with Thor and Ruby I figured this could be documented in the Readme.