Added some notes on the w64 ABI.

Rev: nettle/x86_64/README:1.3

x86_64/README

@@ -23,3 +23,31 @@ Registers	May be		Argument
 %r13		
 %r14		
 %r15		
+
+w64 ABI, according to
+http://msdn.microsoft.com/en-us/library/9z1stfyw.aspx:
+
+Registers	May be		Argument
+		clobbered	number
+
+%rax 		Y
+%rbx		
+%rcx		Y		1
+%rdx		Y		2
+%rbp		
+%rsi
+%rdi
+%rsp		(SP)
+%r8		Y		3
+%r9		Y		4
+%r10		Y
+%r11		Y
+%r12		
+%r13		
+%r14		
+%r15		
+
+Additional arguments are passed on the stack. "backing store" on the
+stack for the four register arguments is also required. %xmm6 and
+%xmm7 are callee-saved. The "long" type is just 32 bits. Nettle
+currently does *NOT* support this ABI.