Updated README

README.md

@@ -2,4 +2,52 @@
 [![Build Status](https://travis-ci.org/dhowden/tag.svg?branch=master)](https://travis-ci.org/dhowden/tag)
 [![GoDoc](https://godoc.org/github.com/dhowden/tag?status.svg)](https://godoc.org/github.com/dhowden/tag)
 
-This package provides basic MP3 (ID3v1,2.{2,3,4}) and MP4 metadata parsing.
+This package provides MP3 (ID3v1,2.{2,3,4}) and MP4 (ACC, M4A, ALAC) metadata detection, parsing and artwork extraction.
+
+Parsed metadata is exported via a single interface (giving a consistent API for all supported metadata formats).
+
+    // Metadata is an interface which is used to describe metadata retrieved by this package.
+    type Metadata interface {
+    	Format() Format
+
+    	Title() string
+    	Album() string
+    	Artist() string
+    	AlbumArtist() string
+    	Composer() string
+    	Year() int
+
+    	Track() (int, int) // Number, Total
+    	Disc() (int, int) // Number, Total
+
+    	Picture() *Picture // Artwork
+
+    	Raw() map[string]interface{} // NB: raw tag names are not consistent across formats.
+    }
+
+## Audio Data Checksum (SHA1)
+
+This package also provides a metadata-invariant checksum for audio files: only the audio data is used to
+construct the checksum.
+
+[http://godoc.org/github.com/dhowden/tag#Sum](http://godoc.org/github.com/dhowden/tag#Sum)
+
+## Example Usage
+
+There are a simple command-line tools which demonstrate basic tag extraction and summing:
+
+    $ go get github.com/dhowden/tag/...
+    $ cd $GOPATH/bin
+    $ ./tag 11\ High\ Hopes.m4a
+    Metadata Format: MP4
+     Title: High Hopes
+     Album: The Division Bell
+     Artist: Pink Floyd
+     Composer: Abbey Road Recording Studios/David Gilmour/Polly Samson
+     Year: 1994
+     Track: 11 of 11
+     Disc: 1 of 1
+     Picture: Picture{Ext: jpeg, MIMEType: image/jpeg, Type: , Description: , Data.Size: 606109}
+
+    $ ./sum 11\ High\ Hopes.m4a
+    2ae208c5f00a1f21f5fac9b7f6e0b8e52c06da29