Improve index.php error message for a missing 'vendor/autoload.php'

index.php

@@ -18,7 +18,11 @@ if (is_file(__DIR__ . '/vendor/autoload.php')) {
     // composer dependency package
     require_once(__DIR__ . '/../../../vendor/autoload.php');
 } else {
-    die("Cannot find 'vendor/autoload.php'. Run `composer install`.");
+    die(
+        "Cannot find 'vendor/autoload.php'. If you're using a composer-based Pico install, run `composer install`. "
+        . "If you're rather trying to use one of Pico's pre-built release packages, make sure to download Pico's "
+        . "latest release package named 'pico-release-v*.tar.gz' (don't download a source code package)."
+    );
 }
 
 // instance Pico