Update README.md

Author: thomasheckmann

README.md

@@ -1,2 +1,4 @@
 # zx81-external-rom-with-display
-ZX81 External ROM interface with partial display logic
+ZX81 External ROM interface with partial display logic. The aim of this interface is to be able to run ZX81 ROMS with alternative character set, as found in different Forth ROMS and the ASZMIC rom, without having the need to open up your ZX81 and replace the internal ROM.
+
+Partial display logic only, as this is what's available at the extension bus.