feat(upgrade): add --releasenotes: show release notes and skip upgrading

cli/app/upgrade.go

@@ -31,6 +31,7 @@ var appUpgradeCommand = cli.Command{
 		internal.NoDomainChecksFlag,
 		internal.DontWaitConvergeFlag,
 		internal.OfflineFlag,
+		internal.ReleaseNotesFlag,
 	},
 	Before: internal.SubCommandBefore,
 	Description: `
@@ -269,6 +270,12 @@ recipes.
 			}
 		}
 
+		if internal.ReleaseNotes {
+			fmt.Println()
+			fmt.Print(releaseNotes)
+			return nil
+		}
+
 		if err := internal.NewVersionOverview(app, deployedVersion, chosenUpgrade, releaseNotes); err != nil {
 			logrus.Fatal(err)
 		}

cli/internal/cli.go

@@ -95,6 +95,16 @@ var OfflineFlag = &cli.BoolFlag{
 	Usage:       "Prefer offline & filesystem access when possible",
 }
 
+// ReleaseNotes stores the variable from ReleaseNotesFlag.
+var ReleaseNotes bool
+
+// ReleaseNotesFlag turns on/off printing only release notes when upgrading.
+var ReleaseNotesFlag = &cli.BoolFlag{
+	Name:        "releasenotes, r",
+	Destination: &ReleaseNotes,
+	Usage:       "Only show release notes",
+}
+
 // MachineReadable stores the variable from MachineReadableFlag
 var MachineReadable bool