Add optional authored Ruins structures to Ages (#91)

README.md

@@ -233,6 +233,9 @@ Data-pack runtime profiles are documented in
 - NeoForge 21.1.264 or newer in the 21.1 line
 - Dynamic Dimensions AR Edition 0.9.1 and Bad Packets 0.8.2 (client and server)
 
+Ruins is optional. When installed, its configured structures have individual
+authorable pages; see [Ruins compatibility](docs/RUINS_COMPATIBILITY.md).
+
 Create is optional. When installed, Rivencraft uses its Ponder renderer for
 book previews when Immersive Portals is absent. Immersive Portals is also
 optional and takes priority in the default `AUTO` mode, providing a live