Add a section on Phone Home mitigations

[dmitrizagidulin]

For a given renderMethod template, a publisher has to make the usual engineering trade-off: link to it, or embed it?

Linking to a template -- space efficient, BUT phone-home possibility.

Embedding a template -- secure, but dramatically increased VC size (depending on size, may have parsing issues on mobile platforms).

Discuss possible mitigation methods:

• Does compression (gzip + text-encode) help any?
• Common template URLs (especially with digest hashes) can be cached by wallets
• Trusted cached directories of templates
• etc

[dmitrizagidulin]

Discussed on Weds call, this is ready for initial PRs.

Would likely be helpful to separate into at least 2-3 sections:

• Linking vs Embedding (effect on offline accessibilty etc)
• Phone home mitigations