Rewrite README to make the problem legible to non-Xcode readers

[maximbilan]

Summary

Feedback was that readers unfamiliar with Xcode internals couldn't tell, from the README alone, what this Action actually solves. Rewrite the top half of the README to fix that without touching the substance below it.

• Lead with the symptom (your weekly SPM-update CI job has gone silent) and an ecosystem analogy (npm update / bundle update / go get -u ./...) instead of naming the broken xcodebuild flag in the first sentence.
• Replace the abstract "Why" section with The problem in one minute — a concrete scenario that explains Package.resolved in passing, so the reader doesn't need prior Xcode knowledge.
• Add a Do I need this? checklist and an explicit "you don't need this if your project is a standalone Package.swift" line.
• Add a How the fix works ASCII diagram so a skimmer can stop reading and still have the mental model.
• Collapse the How it works internally section in a <details> block — present for the curious, out of the way for the first-time reader.
• Usage / Inputs / Outputs / Caveats are unchanged in substance.

Test plan

• Render README on GitHub and confirm the <details> block collapses correctly
• Confirm the ASCII diagram renders inside the fenced code block on both GitHub web and mobile
• Skim from the top as a non-Xcode reader and check the problem lands before any Xcode-specific term is introduced

🤖 Generated with Claude Code