[feat] add example scripts for transcription, model downloading, and updates; improve documentation with guides for CI, packaging, and development

CONTRIBUTING.md

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+# Contributing to PolyScribe
+
+Thanks for your interest in contributing! This guide explains the workflow and the checklist to follow before opening a Pull Request.
+
+Workflow (fork → branch → PR)
+1) Fork the repository to your account.
+2) Create a feature branch:
+- git checkout -b feat/short-description
+3) Make changes with focused commits and good messages.
+4) Run the checklist below.
+5) Push and open a Pull Request against the main repository.
+
+Developer checklist (before opening a PR)
+- Build:
+  - cargo build (preferably without warnings)
+- Tests:
+  - cargo test (all tests pass)
+- Lints:
+  - cargo clippy --all-targets -- -D warnings (fix warnings)
+- Documentation:
+  - Update README/docs for user-visible changes
+  - Update CHANGELOG.md if applicable
+- Tests for changes:
+  - Add or update tests for bug fixes and new features where reasonable
+
+Local development tips
+- Use `cargo run -- <args>` during development.
+- For faster feedback, keep examples in the examples/ folder handy.
+- Keep functions small and focused; prefer clear error messages with context.
+
+Code of conduct
+- Be respectful and constructive. Assume good intent.