DATASOLR-275 - Added code of conduct.

Moved CONTRIBUTING file to Asciidoctor along the way.

CODE_OF_CONDUCT.adoc

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+= Contributor Code of Conduct
+
+As contributors and maintainers of this project, and in the interest of fostering an open and welcoming community, we pledge to respect all people who contribute through reporting issues, posting feature requests, updating documentation, submitting pull requests or patches, and other activities.
+
+We are committed to making participation in this project a harassment-free experience for everyone, regardless of level of experience, gender, gender identity and expression, sexual orientation, disability, personal appearance, body size, race, ethnicity, age, religion, or nationality.
+
+Examples of unacceptable behavior by participants include:
+
+* The use of sexualized language or imagery
+* Personal attacks
+* Trolling or insulting/derogatory comments
+* Public or private harassment
+* Publishing other's private information, such as physical or electronic addresses,
+  without explicit permission
+* Other unethical or unprofessional conduct
+
+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.
+
+By adopting this Code of Conduct, project maintainers commit themselves to fairly and consistently applying these principles to every aspect of managing this project. Project maintainers who do not follow or enforce the Code of Conduct may be permanently removed from the project team.
+
+This Code of Conduct applies both within project spaces and in public spaces when an individual is representing the project or its community.
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting a project maintainer at spring-code-of-conduct@pivotal.io.
+All complaints will be reviewed and investigated and will result in a response that is deemed necessary and appropriate to the circumstances.
+Maintainers are obligated to maintain confidentiality with regard to the reporter of an incident.
+
+This Code of Conduct is adapted from the http://contributor-covenant.org[Contributor Covenant], version 1.3.0, available at http://contributor-covenant.org/version/1/3/0/[contributor-covenant.org/version/1/3/0/].

CONTRIBUTING.adoc

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+= Spring Data contribution guidelines
+
+You find the contribution guidelines for Spring Data projects https://github.com/spring-projects/spring-data-build/blob/master/CONTRIBUTING.adoc[here].