Add developer guide

[pleshakov]

As a potential contributor to the NGINX Kubernetes Gateway
I want a developer guide to explain how to build and test my contributions
So that I do not have to spend an excessive amount of time figuring out how to contribute.

Acceptance

• A developer guide is created within the NKG project that at minimum contains:
  • How to setup the development environment
  • How to build and test the project
  • How to implement a feature
  • A code style guide
  • Information on the architecture and design of the project

Tasks

Beta Give feedback

1. Add developer guide to CONTRIBUTING.md nginx-kubernetes-gateway#670
   documentation +1
   
2. Add code style guide nginx-kubernetes-gateway#671
   +0
   
3. Add enhancement proposal process document nginx-kubernetes-gateway#672
   +0
   
4. Add architecture and design doc nginx-kubernetes-gateway#673
   documentation +1
   

[mpstefan]

Need to add style guide to this story