added contributing docs

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.
+
+## Feature request
+
+If you think a feature is missing, please report it or implement it. If you report it, describe
+precisely what you would like to see implemented. It would be nice if you can do
+some search before submitting it and link the resources to your description.
+
+## Bug report
+
+If you think you have detected a bug or a doc issue, please report it (open [issue][issue]) or even better fix it. If you report it,
+please be the more precise possible. Here a little list of required information:
+
+ * Symfony-standard fork which reproduces the bug.
+ * Precise description of the bug.
+ * Symfony version used.
+ * Bundle version used.
+
+## Bug fix
+
+Fork the repository, clone it and create a new branch with the following commands:
+
+ ``` bash
+ $ git clone [email protected]:liip/LiipImagineBundle.git
+ $ git checkout -b bug-fix-description
+ ```
+
+ Then, install the dependencies through [Composer][composer]
+
+ ``` bash
+ $ composer install
+ ```
+
+ When you're on the new branch with the dependencies, code as much as you want and when the fix is ready,
+ you will need to add tests and update the documentation. Everything is tested with
+ [PHPUnit][php-unit], documentation formatted with [reStructuredText][rst] under the `Resources/doc` directory.
+
+ To run tests, use the following command:
+
+ ``` bash
+ $ phpunit
+ ```
+
+## 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][PSR-2]
+ 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.
+ 3. If you have reworked you patch, please squash all your commits in a single one with the following commands (here, we
+    will assume you would like to squash 3 commits in a single one):
+
+    ``` bash
+    $ git rebase -i HEAD~3
+    ```
+ 4. If your branch conflicts with the master branch, you will need to rebase and repush it with the following commands:
+
+    ``` bash
+    $ git remote add upstream [email protected]:liip/LiipImagineBundle.git
+    $ git pull --rebase upstream master
+    $ git push origin bug-fix-description -f
+    ```
+## Further information
+
+ * [General GitHub documentation][gh-help]
+ * [GitHub pull request documentation][gh-pr]
+
+ [rst]:      http://symfony.com/doc/current/contributing/documentation/format.html#restructuredtext
+ [php-unit]: http://phpunit.de/
+ [composer]: https://getcomposer.org/
+ [gh-help]:  https://help.github.com
+ [gh-pr]:    https://help.github.com/send-pull-requests
+ [issue]:    https://github.com/liip/LiipImagineBundle/issues/new
+ [pr]:       https://github.com/liip/LiipImagineBundle/pull/new
+ [PSR-2]:    https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md