readme: Usage notes

Author: neilpa

README.md

@@ -6,9 +6,71 @@ Basic reading of [JPEG File Interchange Format (JFIF)][wiki-jfif] segments
 
 [wiki-jfif]: https://en.wikipedia.org/wiki/JPEG_File_Interchange_Format#File_format_structure
 
+## Install
+
+Use `go get` to install the module
+
+```
+go get neilpa.me/go-jfif
+```
+
+Or use it and rely on go modules to "do the right thing".
+
 ## Usage
 
-TODO
+Reading JFIF segments from an existing JPEG.
+
+```go
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"neilpa.me/go-jfif"
+)
+
+func main() {
+	f, err := os.Open("path/to/file.jpg")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// See also jfif.ScanSegments which doesn't read the segment payload.
+	// This is used to detect the segment "signatures" for some APPn segments.
+	segs, err := jfif.DecodeSegments(f)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for _, s := range segs {
+		sig, _, _ := s.AppPayload()
+		sig = jfif.CleanSig(sig)
+		fmt.Printf("%d\t%s\t%s\n", s.Length, s.Marker, sig)
+	}
+}
+```
+
+Appending new segments to a JPEG. (Under the hood this edits a copy of the file before renaming the copy
+to finalize the updates).
+
+```go
+package main
+
+import (
+	"log"
+
+	"neilpa.me/go-jfif"
+)
+
+func main() {
+	err := jfif.Add("path/to/file.jpg", jfif.COM, []byte("adding a comment to this file"))
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+```
 
 ## Tools