added contributing info

CONTRIBUTING.md

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+Contribution Guidelines
+=======================
+
+First of all, each single contribution is appreciated, whether a typo fix,
+improved documentation, a fixed bug or a whole new feature.
+
+## Making your changes
+
+ 1. Fork the repository on GitHub
+ 2. Pull requests must be sent from a new hotfix/feature branch, not from `master`.
+ 3. Make your modifications, coding standard for the project is [PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)
+ 4. Commit small logical changes, each with a descriptive commit message.
+    Please don't mix unrelated changes in a single commit.
+
+## Commit messages
+
+Please format your commit messages as follows:
+
+    Short summary of the change (up to 50 characters)
+
+    Optionally add a more extensive description of your change after a
+    blank line. Wrap the lines in this and the following paragraphs after
+    72 characters.
+
+## Submitting your changes
+
+ 1. Push your changes to a topic branch in your fork of the repository.
+ 2. [Submit a pull request][pr] to the original repository.
+    Describe your changes as short as possible, but as detailed as needed for
+    others to get an overview of your modifications.
+
+## Further information
+
+ * [General GitHub documentation][gh-help]
+ * [GitHub pull request documentation][gh-pr]
+
+ [gh-help]: https://help.github.com
+ [gh-pr]:   https://help.github.com/send-pull-requests
+ [issue]:   https://github.com/beryllium/icelus/issues/new
+ [pr]:      https://github.com/helios-ag/FMElfinderBundle/pull/new