Update 00-intro.md

Added note to help install PHP via brew if it's missing.

doc/00-intro.md

@@ -100,6 +100,9 @@ Composer is part of the homebrew-php project.
 2. Run `brew install josegonzalez/php/composer`.
 3. Use Composer with the `composer` command.
 
+> **Note:** If you recieve an error saying PHP53 or higher is missing use this command to install php 
+> `brew install php53-intl`
+
 ## Installation - Windows
 
 ### Using the Installer