Describe changes in CLI's public API for 6.0.0 release

[rosen-vladimirov]

Is your feature request related to a problem? Please describe.
Due to refactoring of CLI's logic for 6.0.0 release, the public API (the one which is available when using NativeScript CLI as a library in a project) has been changed. We need to describe all changes, so the people who rely on the API can migrate easily. Also, it will be great to define common requirements for public API, i.e. which methods/classes are exposed, how are they named, when do we expose them, etc.

Describe the solution you'd like
The Changelog must contain information how to migrate a project using CLI's API from 5.4 to 6.0 release. The CONTRIBUTING.md file must contain information which is the way to expose new method to public API, what are the requirements for it, when it is allowed to change it, etc.

Describe alternatives you've considered
None.

Additional context
All publicly exposed methods are available in the PublicAPI.md file.