Require footing for descriptive book travel

README.md

@@ -36,7 +36,8 @@ independently validates every link.
 Until generated Ages arrive in Milestone 3, written Descriptive Books resolve
 deterministically to the Overworld, Nether, or End according to their Age UUID.
 The Rivencraft creative tab includes one sample book for each. Linking creates
-a bound Return Link to the exact source position and orientation.
+a bound Return Link to the exact source position and orientation. Descriptive
+Book destinations additionally require solid footing beneath the player.
 
 ## Requirements