doc: READMEs for remaining "real world" examples

Updates #14

examples/fnv1a/README.md

@@ -0,0 +1,39 @@
+# fnv1a
+
+[FNV-1a](https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function#FNV-1a_hash) in `avo`.
+
+[embedmd]:# (asm.go /const/ $)
+```go
+const (
+	OffsetBasis = 0xcbf29ce484222325
+	Prime       = 0x100000001b3
+)
+
+func main() {
+	TEXT("Hash64", "func(data []byte) uint64")
+	Doc("Hash64 computes the FNV-1a hash of data.")
+	ptr := Load(Param("data").Base(), GP64v())
+	n := Load(Param("data").Len(), GP64v())
+
+	h := reg.RAX
+	MOVQ(operand.Imm(OffsetBasis), h)
+	p := GP64v()
+	MOVQ(operand.Imm(Prime), p)
+
+	LABEL("loop")
+	CMPQ(n, operand.Imm(0))
+	JE(operand.LabelRef("done"))
+	b := GP64v()
+	MOVBQZX(operand.Mem{Base: ptr}, b)
+	XORQ(b, h)
+	MULQ(p)
+	INCQ(ptr)
+	DECQ(n)
+
+	JMP(operand.LabelRef("loop"))
+	LABEL("done")
+	Store(h, ReturnIndex(0))
+	RET()
+	Generate()
+}
+```