docs: add UPGRADING guide for env var changes

Author: adamweeks

Diff for: UPGRADING.md

@@ -0,0 +1,20 @@
+# Upgrading to Webex
+
+The Cisco Spark SDK has been upgraded and renamed to the Webex SDK.
+With this change, certain environment variables that are passed from the widget to the SDK have been renamed.
+
+
+## Environment variables
+
+**Rename these environment variables**
+
+| old                        | new                   |
+| -------------------------- | --------------------- |
+| `CISCOSPARK_ACCESS_TOKEN`  | `WEBEX_ACCESS_TOKEN`  |
+| `CISCOSPARK_APPID_ORGID`   | `WEBEX_APPID_ORGID`   |
+| `CISCOSPARK_APPID_SECRET`  | `WEBEX_APPID_SECRET`  |
+| `CISCOSPARK_CLIENT_ID`     | `WEBEX_CLIENT_ID`     |
+| `CISCOSPARK_CLIENT_SECRET` | `WEBEX_CLIENT_SECRET` |
+| `CISCOSPARK_LOG_LEVEL`     | `WEBEX_LOG_LEVEL`     |
+| `CISCOSPARK_REDIRECT_URI`  | `WEBEX_REDIRECT_URI`  |
+| `CISCOSPARK_SCOPE`         | `WEBEX_SCOPE`         |