Update testing/install instructions for all controllers

[dholbach]

• Review/extend DEVELOPMENT documentation helm-controller#341
• Review/extend DEVELOPMENT documentation source-controller#454
• Review/extend DEVELOPMENT documentation notification-controller#265
• Review/extend DEVELOPMENT documentation image-automation-controller#237
• Review/extend DEVELOPMENT documentation kustomize-controller#468
• Review/extend DEVELOPMENT documentation image-reflector-controller#182

[stefanprodan]

All these issues should be called "Review/extend DEVELOPMENT documentation" as CONTRIBUTING.md has been deleted from all repos.

[yiannistri]

I would like to work on these issues @kingdonb 🙏

[kingdonb]

Great, thanks @yiannistri I assigned this one to you for tracking. Let me know if there's anything I can help! I spent some time yesterday to try and close out fluxcd/image-reflector-controller#182