Merge pull request #1694 from segmentio/DOC-217_update-ajs-2

DOC-217 add sentence

Author: markzegarelli

src/connections/sources/catalog/libraries/website/javascript/upgrade-to-ajs2.md

@@ -3,7 +3,7 @@ title: Upgrade to Analytics.js 2.0
 strat: ajs
 ---
 
-Analytics.js 2.0 is fully backward compatible with Analytics.js Classic. To upgrade your sources, follow the manual upgrade steps below, or see the schedule for automatic migration.
+Analytics.js 2.0 is fully backward compatible with Analytics.js Classic. To upgrade your sources, follow the manual upgrade steps below, or see the schedule for automatic migration. As with all upgrades, Segment recommends that you start development on a non-production source to test the upgrade process and outcome, prior to upgrading your production sources.
 
 ## Manual upgrade