docs: update readme with CIT spec link

Author: FalsePattern

README.MD

@@ -26,6 +26,7 @@ This mod implements several features based on the OptiFineDoc documentation:
 
 - Connected textures: https://github.com/sp614x/optifine/blob/master/OptiFineDoc/doc/ctm.properties
 - Random mobs: https://github.com/sp614x/optifine/blob/master/OptiFineDoc/doc/random_entities.properties
+- Custom item textures: https://github.com/sp614x/optifine/blob/master/OptiFineDoc/doc/cit_single.properties
 
 Note: the `ft_default` entry in the ctm properties shipped in this mods jar is an internal implementation detail.
 Please ignore it, and do not use it in your resource packs!