Add ReactOS to known-instances.md (#664)

Signed-off-by: Colin Finck <colin@reactos.org>

docs/docs/user/known-instances.md

@@ -48,6 +48,12 @@ This page contains a non-exhaustive list with all websites using Anubis.
   - https://forum.freecad.org/
   - https://wiki.freecad.org/
   </details>
+- <details>
+  <summary>ReactOS</summary>
+  - https://reactos.org/forum
+  - https://reactos.org/wiki
+  - https://git.reactos.org
+  </details>
 - <details>
   <summary>ScummVM</summary>
   - https://bugs.scummvm.org/