Add package README with setup and API docs

[EmilioBejasa]

Summary

• Adds README.md to packages/rn-storybook-auto-screenshots/ for npm consumers
• Covers full setup: JS configuration, native module registration, Android activity, test runner, and screenshot test
• Documents all customization hooks (shouldScreenshotStory, getLoadTimeoutMs, getBootstrapTimeoutMs)
• Includes JS API reference table and "how it works" explanation

Supersedes #32 which was written before BaseStoryScreenshotTest and StorybookRegistry existed.

Test plan

• README renders correctly on GitHub / npm

🤖 Generated with Claude Code