Update README.md

Sigthing client list added

README.md

@@ -42,6 +42,13 @@ A documentation is available [here](https://vulnerability-lookup.readthedocs.io)
 - [JVN iPedia](https://jvndb.jvn.jp), Japan database of vulnerability countermeasure information.
 - [Tailscale](https://tailscale.com/security-bulletins) security bulletins.
 
+## Sighting Sources
+
+Vulnerability-lookup supports the ability to record sightings of vulnerabilities (whether published or unpublished by a source), and a series of sighting clients already exist:
+
+- Fediverse - [A sighting client to gather vulnerability-related information from the Fediverse](https://github.com/CIRCL/FediVuln)
+- MISP - [A sighting client that retrieves vulnerability observations from a MISP server and pushes them to a Vulnerability Lookup instance](https://github.com/MISP/VulnerabilityLookupSighting)
+- Nuclei - [A sighting client designed to retrieve vulnerability-related information from the Nuclei Git repository of templates.](https://github.com/cedricbonhomme/NucleiVuln)
 
 ## Installation