Update CHANGELOG.md

Author: danawillow

CHANGELOG.md

@@ -8,6 +8,7 @@ IMPROVEMENTS:
 * cloudbuild: Add support for substitutions in triggers [GH-1810]
 * compute: Bring regional instance groups up to par with zonal instance groups. [GH-1809]
 * compute: Add labels to Address and GlobalAddress. [GH-1811]
+* container: allow updating node image types [GH-1843]
 
 BUG FIXES:
 * project: Fix bug where app engine wasn't getting enabled on projects that had billing enabled [GH-1795]