README: Link to the Bus document

README.md

@@ -14,6 +14,8 @@ I've been documenting the build process of the various components on my blog at
 ## Modules
 Like the RC2014 I've decided to keep things very modular with each card more or less doing one simple thing, easier to wrap ones head around as a beginner and a lot easier to understand when troubleshooting - also a lot cheaper to upgrade or replace since you don't actually need to replace the entire thing, just the specific board. With the conversion of the RC6502 computer to a SBC-version (Single Board Computer), you now have the option to build an all in one version on a single PCB - alternatively you can build the completely full-sized version as before if you want to! Most can be accomplished with the SBC version so to start things off, I recommend getting PCBs of the SBC ([order](https://www.pcbway.com/project/shareproject/RC6502_Apple_1_SBC__revision_D_.html?inviteid=88707)) as well as the backplane ([order](https://www.pcbway.com/project/shareproject/RC6502_Apple_1_Replica__Backplane_module_revision_A_.html?inviteid=88707)).
 
+The full-sized system uses a 39-pin bus described in [`Bus.md`](Bus.md). The SBC supports the same bus, allowing you to use expansion cards with it; jumpers on the SBC allow you to disable any of the RAM, ROM, PIA and clock so that their functionality can be replaced by peripheral cards on the bus.
+
 In order to build a single board computer (SBC) of RC6502 you'll need the following modules:
 - [RC6502 SBC](https://github.com/tebl/RC6502/tree/master/RC6502%20Apple%201%20SBC) ([order](https://www.pcbway.com/project/shareproject/RC6502_Apple_1_SBC__revision_D_.html?inviteid=88707))