docs: redrafted README for improved developer onboarding

[paulinebanye]

What does this PR do?

This PR updates README.md to provide clearer onboarding and contribution guidance for developers working with ori-sdk-python.

The documentation now explains how to clone the repository, create and push a new branch, install the SDK and development dependencies, use the main SDK features, run the existing quality checks, and contribute changes.

The goal is to make the repository easier for new contributors to set up and understand without changing SDK behaviour or contracts.

Type of change

• feat - new SDK API, model, client, or helper
• fix - bug fix or contract correction
• docs - documentation only
• test - tests only
• refactor - no behavior change
• security - touches signing, validation, install safety, or trust boundaries
• contract-change - changes a cross-repo contract mirrored from ori-specs/runtime

Required checklist

• Linked issue is included below and acceptance criteria are addressed
• pytest -q passes
• mypy ori_sdk tests passes
• ruff check ori_sdk tests passes
• ruff format --check ori_sdk tests passes
• Pre-commit passes for changed files
• Every new .py file has the Apache-2.0 license header
• Public exports in ori_sdk/__init__.py are updated if a public API was added
• Fixtures were added/updated for schema-facing contract changes

The last three items are not applicable.

Contract and compatibility checklist

• This PR mirrors ori-specs or runtime behavior; it does not redefine runtime authority
• If gateway, hub, health, signing, or skill-package contracts changed, the matching repo/spec PR is linked
• Backward compatibility impact is explained, or the issue explicitly allows a breaking change

No gateway, health, signing, skill-package, runtime, or cross-repository contracts are changed by this PR.

Security-sensitive checklist

Complete if this PR touches signing, Hub install, tarball extraction, or skill validation.

• Ed25519 behavior uses cryptography, not PyNaCl, where runtime interoperability matters
• Invalid signatures, malformed payloads, and tampered bytes have negative tests
• Install/extraction paths reject traversal and do not execute skill hook code
• Secrets, tokens, private keys, and raw credentials are never logged or committed

If you used AI assistance

• I can explain every line of AI-generated code in this PR
• I have read and understood every file I modified
• I am not submitting code I cannot defend in review

Related issue

Closes #17

Testing notes

The following checks were run:

pytest -q
mypy ori_sdk tests
ruff check ori_sdk tests
ruff format --check ori_sdk tests
pre-commit run --all-files

This is a documentation-only change. No SDK behaviour, public APIs, fixtures, or contracts were modified.