Add Gitness contributing guide

[dewan-ahmed]

This PR adds the contributing guide for Gitness, which will help new contributors understand the contribution process.

Additionally, it proposes adding the good-first-issue label to issues, enabling new contributors to find relevant work more easily.

Finally, the PR includes the LICENSE section at the end of the README.md file.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

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