Add an overview for the nodeenv Docker image (#447)

Signed-off-by: Mark S. Lewis <[email protected]>

docker/fabric-nodeenv/README.md

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+# Quick reference
+
+- **Maintained by**:  
+  [The Fabric Node chaincode maintainers](https://github.com/hyperledger/fabric-chaincode-node)
+
+# Overview
+
+This image is used by the default [Hyperledger Fabric](https://hyperledger-fabric.readthedocs.io/) chaincode builder when deploying Node (TypeScript or JavaScript) smart contracts. It is not intended for use independently of Hyperledger Fabric.
+
+# Image variants
+
+Detailed information on image tags, interoperability, and supported Node versions can be found in the [compatibility](https://github.com/hyperledger/fabric-chaincode-node/blob/main/COMPATIBILITY.md) documentation.
+
+# License
+
+This image is provided under the [Apache-2.0](https://github.com/hyperledger/fabric-chaincode-node/blob/main/LICENSE) license.