Update README.md (#29)

prisis · Nyholm · commit 8f161ff08a18 · 2018-08-13T07:48:04.000+03:00
diff --git a/README.md b/README.md
@@ -20,7 +20,7 @@
 Via Composer
 
 ``` bash
-$ composer require php-http/psr7-integration-tests
+$ composer require --dev php-http/psr7-integration-tests
 ```
 
 
