doc: post-1.0.0 roadmap for python-tuf

[lukpueh]

Description of issue or feature request:

The CII best practice silver badge requires a roadmap that describes what the project intends to do and not do. We currently provide the following information in order to satisfy the requirement:

The current roadmap is documented as GitHub project "TUF refactor towards 1.0.0" https://github.com/theupdateframework/python-tuf/projects

Note: At the time of revising the badge criteria (January 2022), the project is reaching the end of the current long-term roadmap. Once finalized, a new roadmap will be published.

Regardless of the badge requirement a roadmap seems like a good idea, on an implementation or project level (see positive examples below).

Current behavior: No published plans for after the ongoing project "TUF refactor towards 1.0.0" has been finalized.

Expected behavior: Publish plans for python-tuf for at least the next year once we have released 1.0.0.

positive examples: https://github.com/in-toto/docs/blob/master/ROADMAP.md, https://github.com/in-toto/in-toto/blob/develop/ROADMAP.md negative example: https://github.com/theupdateframework/python-tuf/pull/1698