Add minor update warning to CHANGELOG

CHANGELOG.md

@@ -12,6 +12,10 @@ not have access**
 
 ## [5.15.0](https://github.com/FortAwesome/Font-Awesome/releases/tag/5.15.0) - 2020-09-28
 
+**Minor version upgrade notice: there are some backward-incompatible changes to this release. See the
+[UPGRADING.md guide](https://github.com/FortAwesome/Font-Awesome/blob/master/UPGRADING.md) for more
+information.**
+
 ### Added
 
 * Added guilded, hive, innosoft, instalod, ns8, octopus-deploy, perbyte,