Document how to run select tests matching keyword

Author: zachborboa

README.md

@@ -356,6 +356,13 @@ To run tests:
     $ composer update
     $ ./tests/run.sh
 
+To run select tests:
+
+    $ git clone https://github.com/php-curl-class/php-curl-class.git
+    $ cd php-curl-class/
+    $ composer update
+    $ ./tests/run.sh --filter=keyword
+
 To test all PHP versions in containers:
 
     $ git clone https://github.com/php-curl-class/php-curl-class.git