docs: add a link to the migration guide in the changelog (#1213)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c4a657b..204bb40 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,7 +5,9 @@
 
 ### ⚠ BREAKING CHANGES
 
-* **deps:** require 3.6+ (#961)
+The 2.0 release of `google-api-python-client` is a significant upgrade compared to v1. Please see the [Migration Guide](UPGRADING.md) for more information.
+
+* **deps:** require 3.6+.  (#961)
 
 ### Features