command/0.12checklist: Additional commentary in output

Along with the checklist items, we should also include some surrounding
commentary to add context about what these recommendations are for.

If no changes seem to be required at all, we direct the user back to the
upgrade guide in case there are other steps described there that this
tool is unable to detect or represent.

command/012checklist.go

@@ -70,10 +70,17 @@ func (c *ZeroTwelveChecklistCommand) Run(args []string) int {
 	items := make(map[string][]string)
 	hasItems := c.zeroTwelveChecklists(root, items)
 	if !hasItems {
-		// TODO: Success message
+		fmt.Print(
+			"Looks good! We did not detect any problems that ought to be\naddressed before upgrading to Terraform v0.12.\n\n" +
+				"This tool is not perfect though, so please check the v0.12 upgrade\nguide for additional guidance, and for next steps:\n    https://www.terraform.io/upgrade-guides/0-12.html\n\n",
+		)
 		return 0
 	}
 
+	fmt.Print(
+		"After analyzing this configuration and working directory, we have identified some necessary steps that we recommend you take before upgrading to Terraform v0.12:\n\n",
+	)
+
 	modKeys := make([]string, 0, len(items))
 	for k := range items {
 		modKeys = append(modKeys, k)
@@ -104,6 +111,10 @@ func (c *ZeroTwelveChecklistCommand) Run(args []string) int {
 		}
 	}
 
+	fmt.Print(
+		"Taking these steps before upgrading to Terraform v0.12 will simplify the upgrade process by avoiding syntax errors and other compatibility problems.\n\n",
+	)
+
 	return 1
 }