Updated changelog and documentation (#331)

Author: hiranya911

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased
 
 - [changed] Upgraded Cloud Firestore client to v0.16.0.
+- [changed] Dropped support for Node.js 4.
 
 # v5.13.1
 

README.md

@@ -9,6 +9,7 @@
  * [Installation](#installation)
  * [Contributing](#contributing)
  * [Documentation](#documentation)
+ * [Supported Environments](#supported-environments)
  * [Acknowledgments](#acknowledgments)
  * [License](#license)
 
@@ -52,6 +53,15 @@ about how you can contribute to this project. We welcome bug reports, feature
 requests, code review feedback, and also pull requests.
 
 
+## Supported Environments
+
+We support Node.js 6.0 and higher. Please note that the Admin SDK should only
+be used in server-side/back-end environments controlled by the app developer.
+This includes most server and serverless platforms (both on-premise and in
+the cloud). It is not recommended to use the Admin SDK in client-side
+environments.
+
+
 ## Documentation
 
 * [Setup Guide](https://firebase.google.com/docs/admin/setup/)