add link to generic CONTRIBUTING.md

[atrejut]

Description

add a link to the SAP generic contributing.md in our readme, as required by OSPO.

Checklist

Mark "not applicable" if item on the list does not apply to this pull request.

• I have created/adapted unit tests for new code
  • not applicable
• I have added new dependencies as described at the Contributing page
  • not applicable
• I have made corresponding changes to the documentation (incl. tutorial, etc.)
  • not applicable
• I have provided release notes (in description or as comment) if this PR is a new feature OR a change worth mentioning for next release
  • not applicable
• I have obtained API approvals if a new SAP API or endpoint is used
  • not applicable

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud