Add v3.17.2 to CHANGELOG.md.

[ci skip]

Author: Robert Jackson

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Ember Changelog
 
+### v3.17.2 (March 26, 2020)
+
+- [#18837](https://github.com/emberjs/ember.js/pull/18837) [BUGFIX] Fix `willDestroy` on class based helpers.
+- [#18838](https://github.com/emberjs/ember.js/pull/18838) [BUGFIX] Ensure destructors (e.g. `will-destroy` modifier, `@ember/component`s with `willDestroyElement`, etc) fire when used within an `{{#each}}` block.
+
 ### v3.17.1 (March 22, 2020)
 
 - [#18809](https://github.com/emberjs/ember.js/pull/18809) [BUGFIX] Do not error (RE: `elementId` changing) if `elementId` is not changed