Be more explicit that we are not maintaining the x86 version anymore

README

@@ -1,3 +1,7 @@
+NOTE: we have stopped maintaining the x86 version of xv6, and switched
+our efforts to the RISC-V version
+(https://github.com/mit-pdos/xv6-riscv.git)
+
 xv6 is a re-implementation of Dennis Ritchie's and Ken Thompson's Unix
 Version 6 (v6).  xv6 loosely follows the structure and style of v6,
 but is implemented for a modern x86-based multiprocessor using ANSI C.
@@ -35,8 +39,7 @@ Copyright 2006-2018 Frans Kaashoek, Robert Morris, and Russ Cox.
 
 ERROR REPORTS
 
-We switched our focus to xv6 on RISC-V; see the mit-pdos/xv6-riscv.git
-repository on github.com.
+We don't process error reports (see note on top of this file).
 
 BUILDING AND RUNNING XV6