encoding: add package documentation

Change-Id: Ie908b0b1eff5f61ea589ad375adec0788440f802
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2344
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/doc.go b/encoding/doc.go
new file mode 100644
index 0000000..12c3a28
--- /dev/null
+++ b/encoding/doc.go
@@ -0,0 +1,17 @@
+// Copyright 2019 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package encoding contains subpackages to convert CUE to and from byte-level
+// and textual representations.
+package encoding