Added a contributing file

CONTRIBUTING.md

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+# Contributing to WP Featherlight
+
+:+1::tada: First off, thanks for taking the time to contribute! :tada::+1:
+
+Please make all pull requests against the develop branch. These are the steps required to get up and running with the development version of the plugin:
+
+- Clone forked repo
+- Run `yarn install`
+- Run `bower install`
+- Run `grunt build`
+- Commit all changes and generated files and create a pull request against `develop`
+
+If you run into any issues or have any questions, please open an issue.
+
+Thanks again!