Update CHANGELOG.md

Author: rosbo

CHANGELOG.md

@@ -8,6 +8,7 @@ IMPROVEMENTS:
 BUG FIXES:
 * iam: Fixes issue with consecutive whitespace [GH-1625]
 * compute: Store v1 `self_link` for `(sub)?network` in `google_compute_instance` [GH-1629]
+* compute: `zone` field in `google_compute_disk` should be optional [GH-1631]
 
 ## 1.14.0 (June 07, 2018)