docs: add supported node versions to readme

also, link to v4 migration wiki in v5 migration doc

Author: dpopp07

README.md

@@ -59,6 +59,9 @@ This package has been moved under the name `ibm-watson`. The package is still av
 ## Before you begin
 * You need an [IBM Cloud][ibm-cloud-onboarding] account.
 
+## Prerequisites
+- **Node >=10**: This SDK is tested with Node versions 10 and up. It may work on previous versions but this is not officially supported.
+
 ## Installation
 
 ```sh

UPGRADE-5.0.md

@@ -1,4 +1,7 @@
-# Upgrading to ibm-watson@4.0
+# Upgrading to ibm-watson@5.0
+
+_Note: If migrating from a version less than 4.0, also see the [v4 migration guide wiki](https://github.com/watson-developer-cloud/node-sdk/wiki/v4-Migration-Guide)._
+
 - [Breaking changes](#breaking-changes)
    - [Support for Node v6 and v8 dropped](#support-for-node-v6-and-v8-dropped)