Add OS maintenance strategy to documentation

[MikeMcC399]

Suggestion

Extend the CONTRIBUTING document to describe the strategy for including and adding new images, either by adding text or referring to a separate new document.

This should reference:

• which operating systems are included (currently Alpine and Debian)
• how / when new operating system versions are added
• how / when operating system versions are removed
• how the default operating system version is set and maintained for a given Node.js release line
• how the default Node.js version is set and maintained for a given operating system selection
• how architectures will be added / maintained / removed
• if supported architectures change during the lifecycle of Node.js / operating system versions, describe how these will be handled

Related

#2363

[MikeMcC399]

For Debian, suggestion submitted as #2398

[yosifkit]

More points to consider (which are both bounded by which architectures parent images support)

• how / when architectures are removed
  • For example, when Debian releases move from "release architecture" to "LTS", they usually drop to a more limited set of architectures.
• how / when architectures are added

[MikeMcC399]

@yosifkit

Thanks! I've added your suggestions to the list in a generic form.