docs: add Requirements

Author: kenjis

README.md

@@ -7,6 +7,7 @@ Provides helper traits for PHPUnit.
 
 ## Table of Contents
 
+* [Requirements](#requirements)
 * [Installation](#installation)
 * [Usage](#usage)
     * [TestDouble](#testdouble)
@@ -21,6 +22,10 @@ Provides helper traits for PHPUnit.
         * [$this->getPrivateMethodInvoker()](#this-getprivatemethodinvoker)
 * [License](#license)
 
+## Requirements
+
+- PHP 7.2 or later
+
 ## Installation
 
 Run: