Create SECURITY.md

Proposed security policy (from template https://gitlab.eclipse.org/security/best-practices/)

SECURITY.md

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+# How To Report a Vulnerability
+
+If you think you have found a vulnerability in Eclipse Tycho, you can report it using one of the following ways:
+
+* Contact the [Eclipse Foundation Security Team](mailto:[email protected])
+* Create a [confidential issue](https://gitlab.eclipse.org/security/vulnerability-reports/-/issues/new?issuable_template=new_vulnerability)
+
+You can find more information about reporting and disclosure at the [Eclipse Foundation Security page](https://www.eclipse.org/security/).
+
+# Supported Versions
+
+Latest major version only is supported.
+
+# Security Policy
+
+This project follows [Eclipse Foundation Vulnerability Reporting Policy](https://www.eclipse.org/security/policy/).