Add v3.20.0-beta.1 to CHANGELOG

[ci skip]

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Ember Changelog
 
+### v3.20.0-beta.1 (May 26, 2020)
+
+- [#18926](https://github.com/emberjs/ember.js/pull/18926) / [#18852](https://github.com/emberjs/ember.js/pull/18852) [FEATURE] Implement the [Yieldable Named Blocks](https://github.com/emberjs/rfcs/blob/master/text/0460-yieldable-named-blocks.md) RFC.
+- [#18867](https://github.com/emberjs/ember.js/pull/18867) / [#18927](https://github.com/emberjs/ember.js/pull/18927) / [#18928](https://github.com/emberjs/ember.js/pull/18928) [FEATURE] [Promote `{{in-element}}` to public API](https://github.com/emberjs/rfcs/blob/master/text/0287-promote-in-element-to-public-api.md) RFC.
+- [#18859](https://github.com/emberjs/ember.js/pull/18859) Fix transitionTo with scoped aliased queryParam.
+- [#18961](https://github.com/emberjs/ember.js/pull/18961) [BUGFIX] Remove assertions for updating auto-tracked state without Ember.set / this.set
+
 ### v3.19.0 (May 26, 2020)
 
 - [#18982](https://github.com/emberjs/ember.js/pull/18982) / [#18913](https://github.com/emberjs/ember.js/pull/18913) [BUGFIX] Update rendering engine to latest version.