build: drop support for Node versions 6 and 8

BREAKING CHANGE: This SDK may no longer work with applications running on Node 6 or 8.

Author: dpopp07

UPGRADE-4.0.md

@@ -0,0 +1,7 @@
+# Upgrading to ibm-watson@4.0
+- [Breaking changes](#breaking-changes)
+   - [Support for Node v6 and v8 dropped](#support-for-node-v6-and-v8-dropped)
+
+## Breaking changes
+### Support for Node v6 and v8 dropped
+The SDK no longer supports Node versions 6 and 8, as reflected in the `engines` property in the package.json file. Version 6 reached end of life in April 2019 and Version 8 reaches end of life on 31 December 2019.