Add Code of Conduct to BitShares-Core Repository

**User Story**
As a `BitShares Community Member` we expect our project maintainers and contributors to `follow a Code of Conduct` so that `participation in this project remains 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.

**Additional Context (optional)**
The BitShares project was created to foster an open, innovative and inclusive community to support development of the platform, tools and documentation. To clarify expected behavior within our repositories, we __propose to adopt__ the [Contributor Covenant](http://contributor-covenant.org/). This code of conduct has been adopted by [many other open source communities](http://contributor-covenant.org/adopters/) and feel it expresses our community values well.

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start by reviewing the Contributor Covenant linked in the issue and the repository's existing top-level documentation conventions. Add the adopted Code of Conduct to the repository, then verify that the document clearly states the expected community behavior and that the issue's task is complete.

Written by the indexing model from the issue text.