Add CONTRIBUTING.md

[ceyhunsen]

Proposal Description

Some people, outside of Chainway, wanted/wants to contribute to Clementine. But we don't have any documentation that describes what we allow outsiders to do. This is needed to encourage people to contribute to Clementine.

• Add CONTRIBUTING.md (https://github.com/cezaraugusto/github-template-guidelines/blob/master/.github/CONTRIBUTING.md)
• Update documentation with our rules