Updating the section about security issues

The app is full of security vulns but if there is something undocumented I'd like it mentioned and tracked in the issues section. Because this isn't something that is supporting a production instance of some application, I'm totally fine with the current way of submitting (through issues).

Author: cktricky

CONTRIBUTING.md

@@ -24,9 +24,6 @@ Please go through the project wiki, documentation and source code first,
 or try to ask your question in our
 [Slack Channel](https://owasp.slack.com/messages/C04THC44W).
 
-If you find a security bug, do not report it through GitHub.
-Please send an e-mail to railsgoat@gmail.com instead.
-
 ## Sending Pull Requests
 
 Before sending a new Pull Request, take a look on existing Pull Requests