Add usage message for interactive terminal

src/cli.ts

@@ -1,9 +1,15 @@
 #!/usr/bin/env bun
 
+import { isatty } from "node:tty"
 import { renderMarkdown } from "./index"
 
 const args = process.argv.slice(2)
 
+if (args.length === 0 && isatty(0)) {
+  process.stderr.write(`Usage: tm <file>\n       tm < file\n       command | tm\n`)
+  process.exit(0)
+}
+
 let input: string
 
 if (args.length > 0 && args[0] !== "-") {