Blank Issue Template Description

1st chunk of `.github/ISSUE_TEMPLATE.md`

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<!--
    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.
-->

Title: Blank Issue Template Description

Summary

This text explains that the blank issue template should only be used for extraordinary issues that don't fit the standard issue templates. It advises users to use the specific templates instead unless they are certain their issue is relevant to Nixpkgs and requires a free-form description.