docs: add contributing guidelines

CONTRIBUTING.md

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+# Contributing to Inversify monorepo
+
+## Guidelines
+
+- Please follow [unit testing guidelines](./docs/testing/unit-testing.md) when testing your modules.
+
+- Please wite [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/). Conventional commits are enforced via git hooks and gh actions thanks to [commitlint](https://commitlint.js.org/). The `commit` npm script might be helpful for newcomers.
+
+- Create an issue before sending a PR.
+
+- Feel free to ask for help from other members of the InversifyJS team via the chat / mailing list or github issues.