add one-line description for crypto library

pkg/crypto/lib/crypto.dart

Index: pkg/crypto/lib/crypto.dart
diff --git a/pkg/crypto/lib/crypto.dart b/pkg/crypto/lib/crypto.dart
index f5f737bf8d7c781565f78134300e714a3081681b..ee11c2587e3001b93886e99ac7065596309b72d5 100644
--- a/pkg/crypto/lib/crypto.dart
+++ b/pkg/crypto/lib/crypto.dart
@@ -2,6 +2,10 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+/**
+ * Cryptographic algorithms, with support for hash functions such as
+ * SHA-1, SHA-256, HMAC, and MD5.
+ */
 library crypto;
 
 import 'dart:math';