Update ISSUE_TEMPLATE.md (#8292)

* Update ISSUE_TEMPLATE.md

Customer wanted us to be more explicit on this page about searching for existing tickets in JIRA vs Github before proceeding to submit a new ticket.

* Update ISSUE_TEMPLATE.md

* Shorten proposed text

ISSUE_TEMPLATE.md

@@ -1,9 +1,13 @@
-Per Mattermost guidelines, GitHub issues are for bug reports: http://www.mattermost.org/filing-issues/
+Per Mattermost guidelines, GitHub issues are for bug reports: <http://www.mattermost.org/filing-issues/>.
 
 For troubleshooting see: http://forum.mattermost.org/.
 For feature proposals see: http://www.mattermost.org/feature-requests/
 
-If you've found a bug--something appears unintentional--please report using the following format: 
+If you've found a bug--something appears unintentional--please follow these steps:
+
+1. Confirm you’re filing a new issue. [Search existing tickets in Jira](https://mattermost.atlassian.net/issues/) to ensure that the ticket does not already exist.
+2. Confirm your issue does not involve security. Otherwise, please see our [Responsible Disclosure Policy](https://about.mattermost.com/report-security-issue/).
+3. [File a new issue](https://github.com/mattermost/mattermost-server/issues/new) using the format below. Mattermost will confirm steps to reproduce and file in Jira, or ask for more details if there is trouble reproducing it. If there's already an existing bug in Jira, it will be linked back to the GitHub issue so you can track when it gets fixed. 
 
 #### Summary
 Bug report in one concise sentence