Added Contributing file with information

CONTRIBUTING.md

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+# Contributing
+
+Contributions are **welcome**!
+
+## Procedure 1
+
+1. Look if there is an existing [issue](https://github.com/una/CSSgram/issues) for your case. 
+2. If there isn't an existing issue, then create one.
+
+## Procedure 2
+
+1. Fork this repository
+2. Clone the fork onto your system
+3. `npm install` dependencies (must have Node installed)
+4. Run `gulp` which starts dev server, watches and does compilations to .dev folder
+**In master:**
+**gulp build**: does compilation, files are generated in the respective folders
+**gulp server**: starts server in root to view compiled files after master build
+5. Make changes (e.g. edit the .scss file related to the filter) and check the test site with your changes (see file structure outline below)
+6. Submit a PR referencing the issue with a smile :smile:
+
+Filters are really fun to create! Reference photos created by [Miles Croxford](https://twitter.com/milescroxford) can be found [here](https://instagram.com/cssgram/).
+
+## File Structure Outline
+
+- `source/css/cssgram.css` contains each of the CSS classes you can apply to your `<img>` to give it the filter. You should use `source/css/cssgram.min.css` for production if you want access to all of the library
+- `source/scss/` contains the source files for individual classes and placeholder selectors you can use to extend CSS classes in Sass
+- `site/` is the public facing website
+- `site/test` is how you test filters if you're developing, remember to change `is_done` for the filter you are creating in `site/filters.json`.
+
+Note: This will also have mixin options and a PostCSS Component.
+