Added concrete example document

Author: michellesanver

contributing/code_of_conduct_concrete_example_document.rst

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+Code of Conduct: Concrete Example Document
+==========================================
+This is a living document that serves to give concrete examples to unwanted behaviour. These examples have all taken place somewhere in the PHP community in the past, and are clear code of conduct violations according to the Symfony code of conduct.
+
+Concrete Examples
+-----------------
+- Unwelcome comments regarding a person’s lifestyle choices and practices, including those related to food, health, parenting, drugs, and employment.
+- Deliberate misgendering or use of ‘dead’ (rejected) names
+- Threats of violence like "The person that created this PR should be punched in the face"
+- Incitement of violence towards any individual, including encouraging a person to commit suicide or to engage in self-harm
+- Sustained disruption of discussion
+- Pattern of inappropriate social contact, such as requesting/assuming inappropriate levels of intimacy with others
+- Continued one-on-one communication after requests to cease
+- Putting down people based on their technology choices or their work
+
+The original list is inspired and modified from https://geekfeminism.org/about/code-of-conduct and confirmed by experiences from PHPWomen.