add CONTRIBUTING file to project

CONTRIBUTING.md

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+#Setting up
+**Hint.css** uses [Grunt](http://gruntjs.com/) for the build process which you need to have installed on your system.
+
+Also there are two Grunt tasks required to build the library:
+
+1. grunt-contrib-mincss `npm install grunt-contrib-mincss`
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+2. grunt-sass `npm install grunt-sass`
+
+
+#Building
+Once you have the dependancies installed, run `grunt` from the project directory. This will run the default grunt task which only compiles the SCSS files into `hint.css` file.
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+
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+