compress/zlib: add example to doc comment

Fixes #548.

R=r
CC=golang-dev, graycardinalster
https://golang.org/cl/190062

src/pkg/compress/zlib/reader.go

@@ -2,8 +2,25 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// The zlib package implements reading and writing of zlib
-// format compressed files, as specified in RFC 1950.
+/*
+The zlib package implements reading and writing of zlib
+format compressed data, as specified in RFC 1950.
+
+The implementation provides filters that uncompress during reading
+and compress during writing.  For example, to write compressed data
+to a buffer:
+
+	var b bytes.Buffer
+	w, err := zlib.NewDeflater(&b)
+	w.Write(strings.Bytes("hello, world\n"))
+	w.Close()
+
+and to read that data back:
+
+	r, err := zlib.NewInflater(&b)
+	io.Copy(os.Stdout, r)
+	r.Close()
+*/
 package zlib
 
 import (