docs: add link to contribution guide in README

[braaar]

Description

Adds a new heading for contributing in the readme, which directs you to the existing contribution guideline under Typestack/.github

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• documentation added or updated
• I have run the project locally and verified that there are no errors

Fixes

fixes #1784

[github-actions]

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