Add contributing guidelines

[sormuras]

Add contributing guidelines and other related documents following the example of repositories hosted under https://github.com/oracle-actions

[sormuras]

• https://github.com/oracle-actions/.github/blob/main/CONTRIBUTING.md

[sormuras]

Integrate https://github.com/apps/oracle-contributor-agreement