Document the change from 'return' to 'croak' in Changes, which is apparently new since the 0.17 release

The 0.17 code on CPAN clearly still shows 'return' being used in these
cases, while HEAD in git clearly shows the 'croak' behavior.

Changes

@@ -1,5 +1,13 @@
 Revision history for Selenium-Remote-Driver
 0.18   7-26-2013
+        [THINGS THAT MIGHT BREAK YOUR CODE]
+        - We now 'croak' instead of returning error strings in a number of
+          cases where invalid method calls were made. This should make
+          debugging easier in some cases, and is more aligned with our
+          documentation which already stated that we would croak on errors.
+          Still, your code may depend on the old behavior.
+          https://github.com/aivaturi/Selenium-Remote-Driver/commit/7cdf3dce0fcaa48a883d045368c094351754e85a
+
         [NEW FEATURES]
         - Added a pause() function, that sleeps for the given number of milliseconds, just like
           Test::WWW::Selenium.  https://github.com/aivaturi/Selenium-Remote-Driver/issues/60