Add a note about font display in Mode 7.

Author: zoltanvb

docs/library/b2.md

@@ -109,6 +109,8 @@ Joypad is fully assignable to keyboard keys in core options.
 Keyboard layout of the BBC Master (BBC Micro is the same, apart from cursor key arrangement and the lack of keypad):
 ![](../image/core/b2/bbc-master-keyboard.png)
 
+Keep in mind that certain characters will appear differently in the default Mode 7 prompt, same way as on the original machine. By changing to `MODE 6` the font display can be "fixed".
+
 Most mappings are straightforward positionally from an ISO UK keyboard:
 
 - Dark green: natural mapping, both position and function matches nicely