README: Update for the "menu" -> "category" rename

See flow3r-firmware!571
and api!7

Also, document acceptable values.

README.md

@@ -39,7 +39,13 @@ increase the `metadata.version` field in `flow3r.toml`.  The commit where the
 # The name of your app as displayed in the menu
 name = "Rahix' Nick"
 
-# The submenu where your app should appear
+# The submenu where your app should appear.
+# One of: "Badge", "Music", "Media", "Apps", "Games"
+category = "Badge"
+
+# OPTIONAL: Same as above, for compatibility with older firmware
+# versions that can't handle categories introduced afterwards.
+# One of: "Badge", "Music", "Apps"
 menu = "Badge"
 
 # OPTIONAL: If your app prefers wifi to be off or on when entering.