Add an upgrade guide to main

Author: shrutiburman

UPGRADE.md

@@ -0,0 +1,12 @@
+# Upgrade Guide
+
+_All `MAJOR` version bumps will have upgrade notes posted here._
+
+## [2023-12-05] 7.x.x to 8.x.x
+
+---
+
+* Supported Node.js versions updated
+  * Upgrade to Node.js = [12, 14, 16, lts]
+  * Dropped support for Node.js < 12 ([#1391](https://github.com/sendgrid/sendgrid-nodejs/pull/1391))
+  * Axios security vulnerability upgrade ([#1391](https://github.com/sendgrid/sendgrid-nodejs/pull/1391))