Create a contribution guide for ORAS

[FeynmanZhou]

Currently, we have more contributors onboarded to the ORAS community but they feel hard to get started. It's necessary to prepare a contribution guide for new contributors. Generally, it should include the following sections:

• Communication, proposal, and design
• Developer guide (How to compile, build, and test locally)
• Unit test and E2E test requirement
• GitHub workflow
• Open a Pull Request
• Code review process and criteria

This is long-term work so we can start to prepare some documents in the community repo gradually. I will create a draft file to placeholder some sections above.

cc @Wwwsylvia @qweeah @oras-project/oras-maintainers for collaboration on this work.