Add Sparkle Validator reference (doc-only change)

[dweekly]

This is just a doc-only change that people can validate their appcast.xml with Sparkle Validator. (Free, Open Source, MIT License) It can fit into a GitHub CI/CD workflow or used standalone.

Fixes # (issue)

Misc Checklist

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

There are a wide variety of tests that ship with Sparkle Validator. And it itself provides 60+ tests of a provided appcast.xml file.

macOS version tested: N/A

[zorgiepoo]

Sorry I don't want to add this at this time. generate_appcast is our recommended way of generating feeds and I try to minimize recommending/sanctioning other software/products. On another note, we don't hear a whole lot from folk wanting an official validator or feedback running into ill-formed appcast issues.