Add Discord link to issue template (#14690)

.github/issue_template.md

@@ -2,7 +2,7 @@
 
 Have you read Marlin's Code of Conduct? By filing an Issue, you are expected to comply with it, including treating everyone with respect: https://github.com/MarlinFirmware/Marlin/blob/bugfix-2.0.x/.github/code_of_conduct.md
 
-Do you want to ask a question? Are you looking for support? Please don't post here. Instead please use the Marlin Firmware forum at http://forums.reprap.org/list.php?415 or the Marlin Facebook Group https://www.facebook.com/groups/1049718498464482/.
+Do you want to ask a question? Are you looking for support? Please don't post here. Instead please use the Marlin Firmware forum at http://forums.reprap.org/list.php?415 or the Marlin Facebook Group https://www.facebook.com/groups/1049718498464482/ or the Marlin Discord Server https://discord.gg/n5NJ59y.
 
 Before filing an issue be sure to test the 1.1 and/or 2.0 "bugfix" branches to see whether the issue is already addressed.