impl: add missing links in relatedwork doc (#912)

The reference links for "Threats, Risks, and Mitigations in the Open
Source Ecosystem" and "Binary Authorization for Borg" were missing.

Signed-off-by: Joshua Lock <joshua.lock@uk.verizon.com>

relatedwork.md

@@ -37,3 +37,8 @@ Other takes on provenance and CI/CD:
 
 -   [The Path to Code Provenance](https://medium.com/uber-security-privacy/code-provenance-application-security-77ebfa4b6bc5)
 -   [How to Build a Compromise-Resilient CI/CD](https://www.youtube.com/watch?v=9hCiHr1f0zM)
+
+<!-- Links -->
+
+[Threats, Risks, and Mitigations in the Open Source Ecosystem]: https://github.com/Open-Source-Security-Coalition/Open-Source-Security-Coalition/blob/master/publications/threats-risks-mitigations/v1.1/Threats%2C%20Risks%2C%20and%20Mitigations%20in%20the%20Open%20Source%20Ecosystem%20-%20v1.1.pdf
+[Binary Authorization for Borg]: https://cloud.google.com/docs/security/binary-authorization-for-borg