Clarify the format for layout names

docs/feature_layouts.md

@@ -35,10 +35,12 @@ New names should try to stick to the standards set by existing layouts, and can
 
 ## Supporting a Layout
 
-For a keyboard to support a layout, the variable (`[a-z0-9_]`) must be defined in it's `<keyboard>.h`, and match the number of arguments/keys (and preferably the physical layout):
+For a keyboard to support a layout, the variable must be defined in it's `<keyboard>.h`, and match the number of arguments/keys (and preferably the physical layout):
 
     #define LAYOUT_60_ansi KEYMAP_ANSI
 
+The name of the layout must match this regex: `[a-z0-9_]+`
+
 The folder name must be added to the keyboard's `rules.mk`:
 
     LAYOUTS = 60_ansi