Add Quick Start for TERN

[goneall]

In the outreach team meeting on 7 August, 2023 it was suggested we add a quick start guide for producing SPDX documents from TERN.

[goneall]

@ivanayov @rnjudge - is this something you could help with? See the quick start guide for context and info.

[rnjudge]

@goneall Sure. We have this all documented in the Tern README (for TV and JSON) but I can add something specifically to the SPDX repo.

[goneall]

@rnjudge - I took a look at the README and I think it has all the information, but is structure more feature by feature.

It might be useful to just have a simple walk-through focused on a simple use case of creating an SPDX JSON file for a Docker image. We could then point to the README file for more complete description of the features and options.