README: add link to coreboot-wiki describing why this works

README.md

@@ -184,7 +184,9 @@ CAUTION: THIS IS NOT ENCOURAGED
 On the X230, there are 2 physical "BIOS" chips. The "upper" 4MB
 one holds the actual bios we can generate using coreboot, and the "lower" 8MB
 one holds the rest that you can [modify yourself once](#flashing-for-the-first-time),
-if you like, but strictly speaking, you don't need to touch it at all. What's this "rest"?
+if you like, but strictly speaking, you
+[don't need to touch it at all](https://www.coreboot.org/Board:lenovo/x230#Building_Firmware).
+What's this "rest"?
 Mainly a tiny binary used by the Ethernet card and the Intel Management Engine.
 
 ## Alternatives