Add a Developer Handbook

[rhuss]

This "handbook" is supposed to be a standalone manual which should help to understand the development flow of the client and also should make it easier for newcomers to ramp up.

Describe the feature:

A Developer Handbook should describe:

• How to get started with client development
• Described build.sh in detail
• How dependency are to be updated (see DEVELOPMENT.md should describe how to update dependency versions #58)
• How PR process works
• How E2E work and how they can be run locally
• How the code is organized (roughly)
• What the kn client API actually is
• How we deal with different version of Knative eventing & serving
• Deprecation process
• CLI option convention
  • boolean options (wait, no-wait, ...)
  • array options (traffic, ....)
  • map options (env, label, ..)
• Mock Clients for Unit testing and how to use them

Part of #510

[rhuss]

@abrennan89 I think such kind of documentation is best hosted in the client repo itself so that it is closer to the code itself. wdyt ?

If so, I would keep that issue within this repo and add the docs (markdown or asciidoc) to this client repo.

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[rhuss]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[dsimansk]

/remove-lifecycle stale