Add link to Slack channel in issue template

Summary:
[Learned](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository#configuring-the-template-chooser) that you can link to external resources from the issue template.
[Example](https://github.com/facebook/docusaurus/issues/new/choose) from the Docusaurus repo.

Reviewed By: jcpetruzza

Differential Revision: D58283377

fbshipit-source-id: e9850752c4f723dae87a272304542f11ec2b14c4

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: true
+contact_links:
+  - name: 🥷 Support
+    url: https://erlanger.slack.com/archives/C05KJHDJJUF
+    about: Please use our Slack channel to ask the community for help.