doc: scripts to run embedmd on markdown files

This will help keep READMEs in sync with code.

Also adds a README for the add example.

Updates #14

examples/add/README.md

@@ -0,0 +1,49 @@
+# add
+
+Add two numbers with `avo`.
+
+The [code generator](asm.go) is as follows:
+
+[embedmd]:# (asm.go)
+```go
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+)
+
+func main() {
+	TEXT("Add", "func(x, y uint64) uint64")
+	Doc("Add adds x and y.")
+	x := Load(Param("x"), GP64v())
+	y := Load(Param("y"), GP64v())
+	ADDQ(x, y)
+	Store(y, ReturnIndex(0))
+	RET()
+	Generate()
+}
+```
+
+We use a [`go:generate`](https://blog.golang.org/generate) line to produce the assembly and Go stub files together.
+
+[embedmd]:# (add_test.go go /.*go:generate.*/)
+```go
+//go:generate go run asm.go -out add.s -stubs stub.go
+```
+
+This produces [`add.s`](add.s) as follows:
+
+[embedmd]:# (add.s)
+```s
+// Code generated by command: go run asm.go -out add.s -stubs stub.go. DO NOT EDIT.
+
+// func Add(x uint64, y uint64) uint64
+TEXT ·Add(SB), $0-24
+	MOVQ	x(FP), AX
+	MOVQ	y+8(FP), CX
+	ADDQ	AX, CX
+	MOVQ	CX, ret+16(FP)
+	RET
+```