add bin command to be resolved in a subproject with yarn workspace

[sixertoy]

Hey there, it might be helpful for some of us to have a reference of the package in the node_modules folder

example usage:

"css:build": "$(yarn bin)/node-sass --importer=$(yarn bin)/node-sass-tilde-importer ./src/scss/styles.scss ./lib/styles.css",

instead of:

"css:build": "$(yarn bin)/node-sass --importer=./../node_modules/node-sass-tilde-importer ./src/scss/styles.scss ./lib/styles.css",