Add simple installation instructions to release notes

[Knute Snortum]

I would like to add simple installation instructions to the bottom of the release notes.  This will lower the hurdles for new users directed to the release page.  The Markdown text I would like to add follows:

## Installation

Windows and Mac users should download the appropriate file and double-click it to start the installation process.

* Windows users should download the file ending in `.msi`.

* Mac users on Intel hardware should download the file ending with `-Intel.dmg`. All other Mac users should download the file ending in `.dmg` without the `-Intel` before it.

* Linux users should download the file ending in `.tar.gz` and consult the [installation instructions](https://github.com/frescobaldi/frescobaldi/blob/master/INSTALL.md#installing-the-compressed-tar-file). Developers should follow [this link](https://github.com/frescobaldi/frescobaldi/blob/master/INSTALL.md#developing-frescobaldi).

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Knute Snortum

[Federico Bruni]

I would remove the part for Linux users. They normally know what to do or just wait for the distro/flatpak package update.

I'll keep this in mind for the next release.

Feel free to add this text in the 4.0.5 release page.

Thanks

Federico

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Frescobaldi homepage: http://www.frescobaldi.org/
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Issue tracker: https://github.com/frescobaldi/frescobaldi/issues
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