Update changelog

Author: mkam

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Unreleased
 
+## BREAKING CHANGES
+
+* Updates team token `Description` to be a pointer, allowing for both nil descriptions and empty string descriptions. Team token descriptions and the ability to create multiple team tokens is in BETA, which is EXPERIMENTAL, SUBJECT TO CHANGE, and may not be available to all users, by @mkam [#1088](https://github.com/hashicorp/go-tfe/pull/1088)
+
 # v1.78.0
 
 ## Enhancements