Improve README.txt on how to run Behat tests

Now that we have the excellent documentation written by @cboyden here:

https://drupal.org/node/2271009

I've updated the README.txt to point to that page!

I might be good to include solutions to common troubleshooting issues. From https://drupal.org/node/1783000

Q. Some tests run, but then I see the following Curl error:

[WebDriver\Exception\CurlExec]
Exception has been thrown in "beforeScenario" hook, defined in FeatureContext::beforeScenario()
Curl error thrown for http POST to http://localhost:4444/wd/hub/session$s
couldn't connect to host
You can't just install selenium.

You need to have selenium running on the system that runs your tests for all feature/scenarios tagged with @javascript!
java -jar yourseleniumjarfile.jar

Wow, I haven't encountered any of those errors, but that link is awesome! Thanks for sharing it!

For the time being, I've added a "Troubleshooting" section our Behat documentation and simply linked there. As we encounter more problems when more people try to run our tests, we can build our own set of tips.

BTW, I posted that because I ran into this problem myself. With the help of Panopoly's tests I was able to follow the instructions, get Selenium running and run the tests.

It's was a very cool way to demonstrate the value of Behat to coworkers.

However, it was confusing at first why it didn't just work because I had skipped over the section that called out Selenium as a requirement.

Also it appears that PhpStorm is adding support for Behat but needs help tracking down all the bugs in their current implementation here: http://youtrack.jetbrains.com/issue/WI-6325