Update gateway contributing guide

[bestbeforetoday]

Tasks:

• [x] Refer to GitHub issues rather than Jira for reporting bugs or feature requests.
• [x] Refer to RocketChat, StackOverflow, Fabric mailing list and GitHub discussions for questions.
• [x] Point to any coding practices that might be relevant.
  • Effective Go
  • Use Go "errors" library, not third-party libraries
  • Effective Java
  • Changes include code, tests and documentation
• [x] Test requirements (unit and, where relevant, scenario tests), including testing frameworks that should be used.
  • Java: AssertJ, Mockito
  • Node: Jest
  • Go: testify, gomock
  • Scenario tests: Cucumber
• [x] Relevant issue tags, for example: help wanted, good first issue.
• [x] Review process / expectations?

[denyeart]

@bestbeforetoday Do you envision updates to the Fabric ReadTheDocs contribution guide? Or something in fabric-gateway repo?

[bestbeforetoday]

@bestbeforetoday Do you envision updates to the Fabric ReadTheDocs contribution guide? Or something in fabric-gateway repo?

A contributing guide specific to this repository. Rightly or wrongly, the ReadTheDocs content is entirely focused on core fabric. Even if other repositories like the fabric-gateway repo were mentioned there, there is repository specific information that makes sense to document in the repository.