chore(cli): notify user when reading from stdin

Only if stdin and stderr are both TTY. To inform users input
is expected, instead of appearing to "hang".

pkg/file/readfile.go

@@ -13,6 +13,7 @@ import (
 
 	"dario.cat/mergo"
 	"github.com/kong/go-database-reconciler/pkg/utils"
+	"golang.org/x/term"
 	"sigs.k8s.io/yaml"
 )
 
@@ -67,6 +68,9 @@ func getContent(filenames []string, mockEnvVars bool) (*Content, error) {
 func getReaders(fileOrDir string) (map[string]io.Reader, error) {
 	// special case where `-` means stdin
 	if fileOrDir == "-" {
+		if term.IsTerminal(int(os.Stdin.Fd())) && term.IsTerminal(int(os.Stderr.Fd())) {
+			fmt.Fprintf(os.Stderr, "reading input from stdin...\n")
+		}
 		return map[string]io.Reader{"STDIN": os.Stdin}, nil
 	}