doc: README for sum example

Updates #14

examples/sum/README.md

@@ -0,0 +1,26 @@
+# sum
+
+Sum an array of `uint64`s.
+
+[embedmd]:# (asm.go go /func main/ /^}/)
+```go
+func main() {
+	TEXT("Sum", "func(xs []uint64) uint64")
+	Doc("Sum returns the sum of the elements in xs.")
+	ptr := Load(Param("xs").Base(), GP64v())
+	n := Load(Param("xs").Len(), GP64v())
+	s := GP64v()
+	XORQ(s, s)
+	LABEL("loop")
+	CMPQ(n, operand.Imm(0))
+	JE(operand.LabelRef("done"))
+	ADDQ(operand.Mem{Base: ptr}, s)
+	ADDQ(operand.Imm(8), ptr)
+	DECQ(n)
+	JMP(operand.LabelRef("loop"))
+	LABEL("done")
+	Store(s, ReturnIndex(0))
+	RET()
+	Generate()
+}
+```