Update slack invite link (#38)

* Update slack invite link

Noticed that the website this links to is about to go down and the main selenium website has a proper invite link.

* Update README.rst

README.rst

@@ -43,4 +43,4 @@ Send a question to the `Nerodia-General <https://groups.google.com/forum/#!forum
 Join our Slack channel
 ======================
 
-Slack has created a special slack server for the Selenium and Watir (including Nerodia) communities. You can join our Slack channel by sending Slack your email `here <http://seleniumhq.herokuapp.com/>`_. You will be sent an invitation with instructions on how to join.
+Selenium has created a special slack server for the Selenium and Watir (including Nerodia) communities. You can join our Slack channel `here <https://join.slack.com/t/seleniumhq/shared_invite/zt-vv33sc0w-VKKQop3WDV_lfrLXGGHvDw>`_.