<feature> Add CONTRIBUTING.md doc

Author: shakaran

.github/CONTRIBUTING.md

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+Contribution Guidelines
+=======================
+
+Thank you for considering contributing to this bundle. We welcome any kind of
+contribution, no matter if its huge or small, about documentation or code. We
+also welcome any kind of developers, from experts to people who just started
+working on Open-Source projects.
+
+Requirements
+------------
+
+Before your first contribution, make sure you'll meet these requirements:
+
+ * You have a user account on [GitHub](https://github.com/).
+ * You have installed in your computer a working environment to develop PHP
+   applications.
+ * You have a basic level of English (code, docs and discussions are in English).
+
+All submitted contributions (both code and documentation) adhere implicitly to
+the [Open-Source MIT License][mit-license].
+
+Proposing New Features
+----------------------
+
+We are determined to maintain the original simple and pragmatic philosophy of
+the bundle. This means that we routinely reject any feature that complicates the
+code too much or which doesn't fit in the bundle's philosophy.
+
+That's why **we strongly recommend you** to propose new features by
+[opening a new issue][create-issue] in the repository to discuss about them
+instead of submitting a pull request with the code of the proposed feature.
+
+Reporting Bugs
+--------------
+
+ 1. Go to [the list of AdminThemeBundle issues][adminthemebundle-issues] and look for any
+    existing bug similar to yours.
+ 2. If the bug hasn't been reported yet, [create a new issue][create-issue] and
+    fill in the given issue template.
+
+Sending Pull Requests
+---------------------
+
+This project follows the same contribution workflow used by the Symfony project.
+First you must clone the repository, then create a feature branch and finally,
+submit a pull request via GitHub.
+
+Read the [Symfony contribution guide][sf-contribution] for more details and replace
+`symfony/symfony-docs` by `avanzu/AdminThemeBundle` in every example.
+
+Further information
+-------------------
+
+ * [General GitHub documentation][gh-help]
+ * [GitHub pull request documentation][gh-pr]
+
+[mit-license]: https://opensource.org/licenses/MIT
+[gh-help]: https://help.github.com
+[gh-pr]: https://help.github.com/send-pull-requests
+[adminthemebundle-issues]: https://github.com/avanzu/AdminThemeBundle/issues?utf8=%E2%9C%93&q=is%3Aissue
+[create-issue]: https://github.com/avanzu/AdminThemeBundle/issues/new
+[symfony-standard]: https://github.com/symfony/symfony-standard
+[sf-contribution]: http://symfony.com/doc/current/contributing/documentation/overview.html#your-first-documentation-contribution