Add note in the instructions.append.md

[asadrjhk]

Add a note in the instructions.append.md file that the input and output of the gigasecond function is of type Date.
closes #1564

[github-actions]

Dear asadrjhk

Thank you for contributing to the JavaScript track on Exercism! 💙
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_{Automated comment created by PR Commenter 🤖.}

[junedev]

Thanks! I left some comments how to make the addition adhere to the guidelines we have.

[junedev]

• Put your two new lines before line 3. Line 3 is also a note but yours is more important.
• Wrap gigasecond in backticks, see https://exercism.org/docs/building/markdown/markdown#h-code
• See https://exercism.org/docs/building/markdown/markdown#h-links for how we provide links in the markdown

Currently, the CI pipeline fails because the formatting is not right. See https://github.com/exercism/javascript/blob/main/CONTRIBUTING.md#format for how to run the formatter.

[asadrjhk]

sorry, about that but this time i try to make the PR using my local system, which makes the new pull request, i think i need to close this.