doc: README for data example

Updates #14

examples/data/README.md

@@ -0,0 +1,33 @@
+# data
+
+Constructing data sections in `avo`.
+
+A data section is declared with the `GLOBL` function, and values are specified with `DATA`. The size of the data section will grow as values are added to it.
+
+[embedmd]:# (asm.go go /.*GLOBL\(/ /^$/)
+```go
+	bytes := GLOBL("bytes", RODATA|NOPTR)
+	DATA(0, U64(0x0011223344556677))
+	DATA(8, String("strconst"))
+	DATA(16, F32(math.Pi))
+	DATA(24, F64(math.Pi))
+	DATA(32, U32(0x00112233))
+	DATA(36, U16(0x4455))
+	DATA(38, U8(0x66))
+	DATA(39, U8(0x77))
+```
+
+The `GLOBL` function returns a reference which may be used in assembly code. The following function indexes into the data section just created.
+
+[embedmd]:# (asm.go go /.*TEXT.*DataAt/ /RET.*/)
+```go
+	TEXT("DataAt", "func(i int) byte")
+	Doc("DataAt returns byte i in the 'bytes' global data section.")
+	i := Load(Param("i"), GP64v())
+	ptr := Mem{Base: GP64v()}
+	LEAQ(bytes, ptr.Base)
+	b := GP8v()
+	MOVB(ptr.Idx(i, 1), b)
+	Store(b, ReturnIndex(0))
+	RET()
+```

examples/data/asm.go

@@ -5,13 +5,13 @@ package main
 import (
 	"math"
 
-	"github.com/mmcloughlin/avo"
+	. "github.com/mmcloughlin/avo"
 	. "github.com/mmcloughlin/avo/build"
 	. "github.com/mmcloughlin/avo/operand"
 )
 
 func main() {
-	bytes := GLOBL("bytes", avo.RODATA|avo.NOPTR)
+	bytes := GLOBL("bytes", RODATA|NOPTR)
 	DATA(0, U64(0x0011223344556677))
 	DATA(8, String("strconst"))
 	DATA(16, F32(math.Pi))