Use actual working example in README.md

* Passing $startUrl to the constructor of `Behat\Mink\Driver\Goutte\Client` didn't work.
* example.com doesn't have a "Chat" button to click.
* When it did work, no feedback was given to indicate anything actually happened - this example will print a URL of a download page on php.net
Update sample based on feedback in PR

* Use `->clickLink(...);` instead of `findLink(...)->click()`
* Use `echo ... . PHP_EOL;` instead of printf("%s\n", ...)`
Bit more explanation about composer.json
Merge pull request #49 from rjmunro/patch-1

Use actual working example in README.md