docs: add contributing badge and fix header / build-status image links in README.md

[PeterPedegru]

Added a contributing badge and fixed several README image links to improve presentation and contributor onboarding.

• Added a "Contributing" badge linking to the CONTRIBUTING.md file:
  https://github.com/UXDProtocol/anchor/blob/master/CONTRIBUTING.md

• Replaced the previous (broken or outdated) build status badge with the GitHub Actions workflow badge:
  https://github.com/coral-xyz/anchor/actions/workflows/tests.yaml/badge.svg

• Updated the header image source to a working camo.githubusercontent.com URL to avoid a 404.

Files changed:

• README.md

Current behavior

• The README referenced a build-status image and header image that resulted in broken/404 renders.
• There was no prominent contributing badge linking to the repository CONTRIBUTING.md.

New behavior

• README now shows a working build-status badge sourced from the repository's GitHub Actions.
• README header image URL has been updated to a valid URL to ensure proper rendering.
• A contributing badge has been added and links directly to the CONTRIBUTING.md file to help new contributors find guidelines quickly.

Breaking changes

None — documentation-only update.

Other information / notes for maintainers

• All changes only affect README assets and links.
• If maintainers prefer different badge styles, hosts, or wish to use locally committed images instead of remote URLs, I can adapt the PR accordingly.
• Happy to split these into separate commits/PRs if you prefer one fix per PR.