Add a Developer Handbook

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 #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

Start by reviewing build.sh, the dependency-update context in issue #58, and the E2E and mock-client entry points mentioned in the request. Organize the handbook around the listed development, build, testing, API, versioning, deprecation, and CLI conventions topics; done means each requested area is covered for newcomers and the PR process is explained.

Written by the indexing model from the issue text.