envsubst: Add package doc

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

envsubst/doc.go

@@ -0,0 +1,19 @@
+/*
+Copyright 2024 The Flux 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 envsubst is a Go package for expanding variables in a string using `${var}` syntax.
+// Includes support for bash string replacement functions.
+package envsubst