Document the requirements of a stack

[ekcasey]

Requirements:

Labels:

• io.buildpacks.stack.metadata OR io.buildpacks.stack.id

Env Vars:

• CNB_USER_ID
• CNB_GROUP_ID
• CNB_STACK_ID

Implementation Suggestions:

• Add a new sections to the docs website
• Link from the buildpack/stacks repo

[ameyer-pivotal]

@ekcasey @sclevine I don't see any reference to io.buildpacks.stack.metadata in our code. Am I missing something?

[ekcasey]

@ameyer-pivotal This was originally intended to be played after buildpacks/pack#179 which introduces this label. We can just document it as is and update the documentation when we introduce the new label.

[ameyer-pivotal]

Started this, but ended up setting it down due to other priorities. Work can be picked up on branch: wip-custom-stacks.