Update CHANGELOG.md for new testing framework

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 1.6.0 (Unreleased)
 
+NEW FEATURES:
+* `terraform test`: The previously experimental `terraform test` command has been moved out of experimental. This comes with a significant change in how Terraform tests are written and executed.
+
+    Terraform tests are now written within `.tftest` files, controlled by a series of `run` blocks. Each `run` block will execute a Terraform plan or apply command against the Terraform configuration under test and can execute conditions against the resultant plan and state.
+
 ENHANCEMENTS:
 * config: Terraform can now track some additional detail about values that won't be known until the apply step, such as the range of possible lengths for a collection or whether an unknown value can possibly be null. When this information is available, Terraform can potentially generate known results for some operations on unknown values. This doesn't mean that Terraform can immediately track that detail in all cases, but the type system now contains the facility for that and so over time we will improve the level of detail generated by built-in functions, language operators, Terraform providers, etc. [GH-33234]
 * jsonplan: Added `errored` field to JSON plan output, indicating whether a plan errored. [GH-33372]