Added basic docs for download command

README.md

@@ -1,11 +1,33 @@
 # 🥧 PIE (PHP Installer for Extensions)
 
-(to be completed...)
+You will need PHP 8.1 or newer to run PIE, but PIE can install an extension to any installed PHP version.
 
 ## Installing
 
 ## Usage
 
+You can download an extension ready to be built or installed using the `download` command. For example, to download the
+`example_pie_extension` extension, you would run:
+
+```shell
+$ bin/pie download asgrim/example-pie-extension
+You are running PHP 8.3.7
+Target PHP installation: 8.3.7 (from /usr/bin/php8.3)
+Platform: NonWindows, x86_64, NonThreadSafe
+Found package: asgrim/example-pie-extension:1.0.1 which provides ext-example_pie_extension
+Extracted asgrim/example-pie-extension:1.0.1 source to: /tmp/pie_downloader_6645f07a28bec9.66045489/asgrim-example-pie-extension-769f906
+$ 
+```
+
+If you are trying to install an extension for a different version of PHP, you may specify this on non-Windows systems
+with the `--with-php-config` option like:
+
+```shell
+bin/pie download --with-php-config=/usr/bin/php-config7.2 my/extension
+```
+
+Windows TBD
+
 ## Developing
 
 ### Testing

src/Command/DownloadCommand.php

@@ -69,6 +69,8 @@ public function execute(InputInterface $input, OutputInterface $output): int
             $phpBinaryPath = PhpBinaryPath::fromPhpConfigExecutable($withPhpConfig);
         }
 
+        // @todo Support Windows using `--with-php-path="C:\usr\php7.4.33"`
+
         $targetPlatform = TargetPlatform::fromPhpBinaryPath($phpBinaryPath);
 
         $output->writeln(sprintf('<info>You are running PHP %s</info>', PHP_VERSION));