crypto/hkdf: add package doc comment

For #61477

Change-Id: I3d3ebf573a21f1f56edfffb3fea53c0b5cbfccd8
Reviewed-on: https://go-review.googlesource.com/c/go/+/634095
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: ianlancetaylor

src/crypto/hkdf/hkdf.go

@@ -2,6 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Package hkdf implements the HMAC-based Extract-and-Expand Key Derivation
+// Function (HKDF) as defined in RFC 5869.
+//
+// HKDF is a cryptographic key derivation function (KDF) with the goal of
+// expanding limited input keying material into one or more cryptographically
+// strong secret keys.
 package hkdf
 
 import (