Support horizontal portals and visible receptacle books (#93)

README.md

@@ -118,6 +118,11 @@ book or page is placed on it; this in-world interaction has no JEI guide.
 With Create installed, Crystal Blocks and Book Receptacles have an optional Ponder scene
 showing crystal-frame construction, book activation, and a parrot crossing the portal. Use the Ponder shortcut
 while hovering either item; Rivencraft does not require Create to run.
+Crystal frames may stand vertically or lie flat: a 3×3 Crystal Block ring with
+an empty center forms a horizontal portal when a Book Receptacle is attached to
+the top or underside of one of its blocks. Insert a bound Linking or Descriptive
+Book to activate it; the book remains visible on the receptacle. A Linking Book aimed at its
+own dimension still opens a visual portal, but travel requires Intra Linking.
 
 Craft a Book Binder from three iron ingots and five planks. Put leather or an
 empty Notebook in the left cover slot. Enter a title, then place a Link Panel