Update CHANGELOG for #38 to include KeysAre() function

Author: bflad

.changelog/38.txt

@@ -1,3 +1,3 @@
 ```release-note:feature
-Introduced `mapvalidator` package with `ValuesAre()` validation function
+Introduced `mapvalidator` package with `KeysAre()` and `ValuesAre()` validation functions
 ```