Update CHANGELOG.md

Changelog update 1.12 (GoogleCloudPlatform#428)

Update Release Notes for April Release (GoogleCloudPlatform#422)

Update changelog for 1.10.0 release (GoogleCloudPlatform#408)

Update changelog for 1.9.0 release (GoogleCloudPlatform#388)

Update changelog for 1.8.0 release (GoogleCloudPlatform#374)

* Update changelog for 1.8.0 release

* update syntax

* a few spacing cleanups

Update changelog 1.7.1 (GoogleCloudPlatform#351)

* update changelog 1.7.1

* update

* colon

Update changelog 1.7.1 (GoogleCloudPlatform#351)

* update changelog 1.7.1

* update

* colon

Adding changelog entries for v1.7 (GoogleCloudPlatform#316)

* Adding changelog entries for v1.7

* Removing unshipped feature

Updating changelog for 1.6.0 release (GoogleCloudPlatform#302)