Explain composer

Author: martin-helmich

README.md

@@ -23,6 +23,8 @@ There are several release branches of this library, each of these being compatib
 | v2 (branch `v2`) | :no_entry_sign: | :no_entry_sign: | :white_check_mark: | :no_entry_sign: |
 | v3 (branch `master`) | :no_entry_sign: | :no_entry_sign: | :no_entry_sign: | :white_check_mark: |
 
+When you are using `composer require` and have already declared a dependency to `phpunit/phpunit` in your `composer.json` file, Composer should pick latest compatible version automatically.
+
 ## Usage
 
 ### Using the `Psr7Assertions` trait