Inside the <Description> tag, add a compatibility section:

<!-- Nightingale -->
<em:targetApplication>
  <Description>
    <em:id>nightingale@getnightingale.com</em:id>
    <em:minVersion>1.11.0a</em:minVersion>
    <em:maxVersion>2.0a</em:maxVersion>
  </Description>
</em:targetApplication>

We want the add-on to be compatible starting with the first 1.11 builds and up to 2.0a (alpha builds of the next nightingale, which does not yet build).

For more information about install.rdf, refer to this article.

You can put the add-on into a folder within your profile directory so you don't have to zip changes everytime, the Building an Extension Article describes the process a bit.

• Port the add-on as above, then test it. If you can, fix issues related to it.
• If there are still major issues, ask in the forums or IRC for help at porting the add-on.
• Alternatively, if the add-on is usable in its core features, it might be worth uploading it with a description of what works and doesn't work.
• Login at Nightingale's wiki, navigate to the add-on page and select “Edit”.
• Use the “Media Files” link to upload the modified add-on, and create the add-on's entry. Please match the style and add a suitable edit description.
• Save the page.