Add v3.18.0-beta.1 to CHANGELOG

[ci skip]

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Ember Changelog
 
+### v3.18.0-beta.1 (March 5, 2020)
+
+- [#18774](https://github.com/emberjs/ember.js/pull/18774) [BUGFIX] Suspend observer deactivation during property changes
+- [#18785](https://github.com/emberjs/ember.js/pull/18785) Drop Node 8 support.
+
 ### v3.17.0 (March 4, 2020)
 
 - [#18688](https://github.com/emberjs/ember.js/pull/18688) / [#18621](https://github.com/emberjs/ember.js/pull/18621) / [#18714](https://github.com/emberjs/ember.js/pull/18714) / [#18743](https://github.com/emberjs/ember.js/pull/18743) / [#18762](https://github.com/emberjs/ember.js/pull/18762) Upgrades Glimmer VM to 0.47.9, fixes ignored `checked` attribute on `input`, fixes using `array` and `hash` helper together