Let's create a guide for developers

[t-kikuc]

What I want

I want a development guide for PipeCD developers, which expands Contributor Guide and CONTRIBUTING.md.

The contents may be like:

• Understanding code & architecture (of each feature)
• How to set up a development environment
• How to debug/test
• Procedure of contribution

cf.

• TiDB Development Guide
• ArgoCD Developer Guide (in Docs)

Why I want

• To develop faster (especially for new developers)
• To increase the number of developers
• To save time in reviewing PRs for reviewers

Background: current problems

• For developers
  • It's difficult to understand the internal mechanism (especially the event-driven architecture)
  • It's troublesome to set up a dev/test environment.
• For reviewers
  • They sometimes need to review PRs of features that they're not familiar with. It takes a long time.

Things to discuss 🗣️

• Intended readers and objectives (MUST BE CLEAR)

  Reader	Objective
  New contributors (including doc fix?)	Contribute in a correct way
  New developers	[1] Understand PipeCD's mechanism [2] Set up for dev&test
  Developers who try developing other new features	Understand the mechanism of each feature
  Reviewers	Understand the mechanism of each feature

• How to publish?
  • Web site (which domain?)
  • markdowns in GitHub (I think this is enough)
• Any other things

[t-kikuc]

Procedure of contribution

This should be described mainly in the main repository.