ISSUE_TEMPLATE.md: clear the blank issue template

This changes the blank issue template to a simple HTML comment telling
users to use the forms instead, while still allowing experienced
contributors to create free-form issues.

Signed-off-by: Fernando Rodrigues <[e-mail]>

.github/ISSUE_TEMPLATE.md

···
-## Issue description
-
-
-
-### Steps to reproduce
-
-
-
-## Technical details
-
-<!-- Please insert the output of running `nix-shell -p nix-info --run "nix-info -m"` below this line -->
+<!--
+    Please note: This blank issue template is meant for extraordinary issues
+    that do not fit the templates. Unless you know your issue is relevant to
+    Nixpkgs and requires the free-form blank issue, please use the issue
+    templates instead.
+-->