VERSIONS.md: document Rock-solid curl releases

While they are not curl releases done by the Open Source project or
provided openly, these version numbers might appear in the wild so let's
document them in this separate list.

Closes #22636

docs/VERSIONS.md

@@ -344,3 +344,14 @@ same version numbers.
 - 0.3: February 1 1997
 - 0.2: December 17 1996
 - 0.1: November 11 1996
+
+## Rock-solid curl
+
+The long-term support releases of curl, called [Rock-solid
+curl](https://rock-solid.curl.dev/), are managed separately and are for paying
+customers only. They are made as releases branches off previous standard curl
+releases, and use version numbers that were never used by the freely available
+version.
+
+- Rock-solid curl 8.18.1: February 25, 2026
+- Rock-solid curl 8.18.2: September 2, 2026