Create CONTRIBUTING.md

.github/CONTRIBUTING.md

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+Contribution Policy for MagicMirror²
+====================================
+
+Thanks for contributing to MagicMirror²!
+
+We hold our code to standard, and these standards are documented below. 
+
+First, before you run the linters, you will need to install them all **and** install the development dependencies:
+
+```bash
+(sudo) npm install -g jscs stylelint html-validator-cli
+npm install
+```
+
+### JavaScript: Run JSCS
+
+We use [JSCS](http://jscs.info) on our JavaScript files.
+
+Our JSCS configuration is in our .jscsrc file.
+
+To run JSCS, use `npm run jscs`.
+
+### CSS: Run StyleLint
+
+We use [StyleLint](http://stylelint.io) to lint our CSS. Our configuration is in our .stylelintrc file.
+
+To run StyleLint, use `npm run stylelint`.
+
+### HTML: Run HTML Validator
+
+We use [NU Validator](https://validator.w3.org/nu) to validate our HTML. The configuration is in the command in the package.json file.
+
+To run HTML Validator, use `npm run htmlvalidator`.