Move all documentation to README.adoc

This includes both separate .md files, and documentation that was on the
head of the .S source files.

Retest everything as this was done, and fix a few easy things.

lidt0.S

@@ -0,0 +1,14 @@
+#include "common.h"
+BEGIN
+    CLEAR
+    movw $handler, 0
+    mov %cs, 2
+    movw $4, 8
+    movl $0, 0xA
+    lidt 8
+    int $0
+    PUTC $'b
+    hlt
+handler:
+    PUTC $'a
+    iret