Adds caching proxy example using HOMEBREW_ARTIFACT_DOMAIN

[colindean]

I brought this up several months ago and got a solution working that's been running on my work machine since early September without failure.

Possible additions:

• A statement expressing any limitations on supporting this configuration
• A statement, perhaps in a comment, limiting the tip to the idea and that it should not be an ever-growing HOWTO for every artifact hosting system.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

[MikeMcQuaid]

Makes sense, thanks @colindean!