Add acknowledgment

Author: hfrick

NEWS.md

@@ -4,6 +4,8 @@
 
 * Started moving error messages to cli (#499, #502).
 
+* `vfold_cv()` and `clustering_cv()` now error on implicit leave-one-out cross-validation (@seb09, #527).
+
 ## Bug fixes
 
 * `vfold_cv()` now utilizes the `breaks` argument correctly for repeated cross-validation (@ZWael, #471).