Create CONTRIBUTE.md

Contribution guideline added

Author: afrasiyabhaider

CONTRIBUTE.md

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+# Contributing
+
+Contributions are **welcome** and will be fully **credited**. We accept contributions via Pull Requests on [Github](https://github.com/spatie/laravel-medialibrary).
+
+## Pull Requests
+
+- **[PSR-4 Coding Standard.]** The easiest way to apply the conventions is to install [PHP CS Fixer](https://github.com/FriendsOfPHP/PHP-CS-Fixer).
+- **Document any change in behaviour.** Make sure the `README.md` and any other relevant documentation are kept up-to-date.
+- **Create feature branches.** Don't ask us to pull from your master branch.
+- **One pull request per feature.** If you want to do more than one thing, send multiple pull requests.
+- **Send coherent history.** Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash them](http://www.git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages) before submitting.