[TASK] Start the contributor guidelines with the CoC

This is the first part of #489.

I'd like to add sections bit by bit to make reviewing easier and
faster, and to allow us to get things online quickly.

This file will be in sync with its counterpart in the Emogrifier
project as much as possible.

Author: oliverklee

.gitattributes

@@ -3,6 +3,7 @@
 /.github/ export-ignore
 /.gitignore export-ignore
 /.phive/ export-ignore
+/CODE_OF_CONDUCT.md export-ignore
 /bin/ export-ignore
 /config/ export-ignore
 /phpunit.xml export-ignore

.github/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing to PHP-CSS-Parser
+
+Those that wish to contribute bug fixes, new features, refactorings and
+clean-up to PHP-CSS-Parser are more than welcome.
+
+When you contribute, please take the following things into account:
+
+
+## Contributor Code of Conduct
+
+Please note that this project is released with a
+[Contributor Code of Conduct](../CODE_OF_CONDUCT.md). By participating in this
+project, you agree to abide by its terms.

CODE_OF_CONDUCT.md

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+# Contributor Code of Conduct
+
+## Our Pledge
+
+In the interest of fostering an open and welcoming environment, we as
+contributors and maintainers pledge to making participation in our project and
+our community a harassment-free experience for everyone, regardless of age,
+body size, disability, ethnicity, gender identity and expression, level of
+experience, nationality, personal appearance, race, religion, or sexual
+identity and orientation.
+
+## Our Standards
+
+Examples of behavior that contributes to creating a positive environment
+include:
+
+* Using welcoming and inclusive language
+* Being respectful of differing viewpoints and experiences
+* Gracefully accepting constructive criticism
+* Focusing on what is best for the community
+* Showing empathy towards other community members
+
+Examples of unacceptable behavior by participants include:
+
+* The use of sexualized language or imagery and unwelcome sexual attention or
+  advances
+* Trolling, insulting/derogatory comments, and personal or political attacks
+* Public or private harassment
+* Publishing others' private information, such as a physical or electronic
+  address, without explicit permission
+* Other conduct which could reasonably be considered inappropriate in a
+  professional setting
+
+
+## Our Responsibilities
+
+Project maintainers are responsible for clarifying the standards of acceptable
+behavior and are expected to take appropriate and fair corrective action in
+response to any instances of unacceptable behavior.
+
+Project maintainers have the right and responsibility to remove, edit, or
+reject comments, commits, code, wiki edits, issues, and other contributions
+that are not aligned to this Code of Conduct, or to ban temporarily or
+permanently any contributor for other behaviors that they deem inappropriate,
+threatening, offensive, or harmful.
+
+## Scope
+
+This Code of Conduct applies both within project spaces and in public spaces
+when an individual is representing the project or its community. Examples of
+representing a project or community include using an official project e-mail
+address, posting via an official social media account, or acting as an
+appointed representative at an online or offline event. Representation of a
+project may be further defined and clarified by project maintainers.
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be
+reported by contacting the project team at (emogrifier at myintervals dot com).
+All complaints will be reviewed and investigated and will result in a response
+that is deemed necessary and appropriate to the circumstances. The project team
+is obligated to maintain confidentiality with regard to the reporter of an
+incident. Further details of specific enforcement policies may be posted
+separately.
+
+Project maintainers who do not follow or enforce the Code of Conduct in good
+faith may face temporary or permanent repercussions as determined by other
+members of the project's leadership.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant][homepage],
+version 1.4, available at
+[http://contributor-covenant.org/version/1/4/][version].
+
+[homepage]: http://contributor-covenant.org
+[version]: http://contributor-covenant.org/version/1/4/