Add contributing guide

CONTRIBUTING.md

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+# Contributing to Phug
+
+## Code of Conduct
+
+We worked hard to provide this pug port. And we get any profit of it. That's why we ask you to be polite and respectful. For example, when you report an issue, please use human-friendly sentences ("Hello", "Please", "Thanks", etc.)
+
+## Issue Contributions
+
+Please report any security issue or risk by emailing [email protected]. Please don't disclose security bugs publicly until they have been handled by us.
+
+For any other bug or issue, please click this link and follow the template if applicable:
+[Create new issue](https://github.com/phug-php/phug/issues/new)
+
+This template will help your provide us the informations we need for most issues (the PHP and/or Pug code you use, the expected behaviour and the current behaviour).
+
+## Code Contributions
+
+Fork the [GitHub project](https://github.com/phug-php/phug) and chek out your copy locally:
+
+```shell
+git clone https://github.com/<username>/phug.git
+cd phug
+git remote add upstream https://github.com/phug-php/phug.git
+```
+Replace `<username>` with your GitHub username.
+
+Then, you can work on the master or create a specific branch for your development:
+
+```shell
+git checkout -b my-feature-branch -t origin/master
+```
+
+You can now edit the "phug" directory contents.
+
+Before committing, please set your name and your e-mail (use the same e-mail address as in your GitHub account):
+
+```shell
+git config --global user.name "Your Name"
+git config --global user.email "[email protected]"
+```
+
+The ```--global``` argument will apply this setting for all your git repositories, remove it to set only your phug fork with them.
+
+Now you can index and commit your modifications as you usually do with git:
+
+```shell
+git add --all
+git commit -m "The commit message log"
+```
+
+If your patch fixes an open issue, please insert ```#``` immediately followed by the issue number:
+
+```shell
+git commit -m "#21 Fix this or that"
+```
+
+Use git rebase (not git merge) to sync your work from time to time:
+
+```shell
+git fetch origin
+git rebase origin/master
+```
+
+Please add some tests for bug fixes and features (for example a pug file and the expected HTML file in /tests/templates), then check all is right with phpunit:
+
+Install PHP if you have'nt yet, then install composer:
+https://getcomposer.org/download/
+
+Update dependencies:
+```
+./composer.phar update
+```
+
+Or if you installed composer globally:
+```
+composer update
+```
+
+Then call phpunit:
+```
+./vendor/bin/phpunit
+```
+
+Make sure all tests succeed before submit your pull-request, else we will not be able to merge it.
+
+Push your work on your remote GitHub fork with:
+```
+git push origin my-feature-branch
+```
+
+Go to https://github.com/yourusername/phug and select your feature branch. Click the 'Pull Request' button and fill out the form.
+
+We will review it within a few days. And we thank you in advance for your help.