Added a warning about the unzip command (symfony#120)

* Added a warning about the `unzip` command (closes symfony#54)

* Rewording to make the warning easier to find with the search engines

* Clarifying the warning about the `unzip` command (concerns the *nix systems only)

* Fixed missing backtick

Author: Deuchnord

README.md

@@ -30,6 +30,9 @@ Use [Composer](https://getcomposer.org/) to install Panther in your project. You
 
     composer req --dev symfony/panther
 
+**Warning:** On \*nix systems, the `unzip` command must be installed or you will encounter an error similar to `RuntimeException: sh: 1: exec: /app/vendor/symfony/panther/src/ProcessManager/../../chromedriver-bin/chromedriver_linux64: Permission denied` (or `chromedriver_linux64: not found`).
+The underlying reason is that PHP's `ZipArchive` doesn't preserve UNIX executable permissions.
+
 ## Basic Usage
 
 ```php