Update issue templates

Author: Tsche

.github/ISSUE_TEMPLATE/-resource--discord.md

@@ -0,0 +1,20 @@
+---
+name: "[Resource] Discord"
+about: Suggest to add a discord community to the list
+title: Add Discord community <community name>
+labels: 'resource: discord'
+assignees: ''
+
+---
+
+**Community name**
+The name of the community.
+
+**Community description**
+A brief description of this community.
+
+**Invite link**
+An invite link to this community. This must not expire and **cannot be a vanity link**.
+
+**Why should this discord community be added?**
+Explain briefly why you think this community is a good fit for this resource site. Why is it relevant to C++?