docs: clarify Runtime Viewer installation and source discovery

[mvanhorn]

Issue #83 reports an empty initial experience after installing an unofficial Homebrew cask, registering the helper service, and expecting applications to appear as connection targets. The maintainer clarified that Runtime Viewer is not published through Homebrew and directed users to GitHub Releases, but the README's Getting Started section currently begins with helper registration and never states how to install the main app. It also does not clearly distinguish the bundled local runtime source, explicitly attached processes, and other Runtime Viewer instances discovered over Bonjour. This documentation gap makes helper installation look like the step that should populate a list of applications.

Summary

Add a concise installation subsection at the start of README.md's Getting Started section that identifies GitHub Releases as the official binary source, tells users to install the macOS app from the release archive, and explicitly notes that the project does not publish a Homebrew cask. Keep the existing helper instructions, but clarify that helper registration enables inter-process communication and injection rather than installing the main app or automatically creating connection targets. Add a short first-use explanation of the three supported entry paths already represented in the README and app: browse the local runtime/framework directory, use Attach Process when its SIP prerequisite is met, or select another Runtime Viewer instance discovered on the local network.

Test plan

• Follow the Getting Started flow from a clean macOS setup and verify that the official Releases link leads to the macOS archive before helper registration is mentioned.
• Verify that the instructions do not imply the helper populates application targets and clearly identify local browsing, explicit process attachment, and Bonjour discovery as separate ways to obtain content.
• Verify that Homebrew is described only as unsupported/unofficial, without supplying or endorsing a cask command the maintainer does not control.
• Check every added relative link and heading anchor in GitHub's rendered Markdown, and confirm the revised text remains consistent with the macOS 15+ requirement and SIP limitation already documented in the README.

Fixes #83