minor symfony#12151 Added the Code of Conduct file (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Added the Code of Conduct file

We were already complying with the Symfony's Code of Conduct in this repository ... but this file was missing. Adding it allows GitHub to display it in some places (e.g. when creating an issue or PR). This is the same we do in the main Symfony repository (https://github.com/symfony/symfony/blob/3.4/CODE_OF_CONDUCT.md).

Commits
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8f799bc Added the Code of Conduct file

Author: javiereguiluz

CODE_OF_CONDUCT.md

@@ -0,0 +1,83 @@
+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, ethnic origin, gender identity and expression, level of
+experience, education, socio-economic status, nationality, personal appearance,
+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
+--------------------
+
+[CoC Active Response Ensurers, or CARE][1], 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.
+
+CARE team members 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 CARE team members.
+
+Enforcement
+-----------
+
+Instances of abusive, harassing, or otherwise unacceptable behavior
+[may be reported][2] by contacting the [CARE team members][1].
+All complaints will be reviewed and investigated and will result in a response
+that is deemed necessary and appropriate to the circumstances. The CARE team is
+obligated to maintain confidentiality with regard to the reporter of an
+incident. Further details of specific enforcement policies may be posted
+separately.
+
+CARE team members who do not follow or enforce the Code of Conduct in good
+faith may face temporary or permanent repercussions as determined by the
+[core team][3].
+
+Attribution
+-----------
+
+This Code of Conduct is adapted from the [Contributor Covenant version 1.4][4].
+
+[1]: https://symfony.com/doc/current/contributing/code_of_conduct/care_team.html
+[2]: https://symfony.com/doc/current/contributing/code_of_conduct/reporting_guidelines.html
+[3]: https://symfony.com/doc/current/contributing/code/core_team.html
+[4]: https://www.contributor-covenant.org/version/1/4/code-of-conduct.html