chore: added note about aur package to readme

[mroetsc]

Hey there, enjoying the project. I've added a package to the AUR:
https://aur.archlinux.org/packages/envelope-bin

which makes it easier to Arch users to install this. It packages the x86_64-unknown-linux-musl.tar.gz binary.

This PR just adds the relevant install instructions to the readme. Please let me know if you want anything changed.

PS: this also closes #28

[mattrighetti]

Thanks for the PR @mroetsc! Do we need anything else to keep that up-to-date?

[mroetsc]

Not really, the package needs to be bumped when you create a release here. That process includes:

• updating the version numbers in the PKBUILD (the file format used for aur packages)
• updating the shas of the files
  you can view the PKBUILD here, the syntax is pretty straight forward.

If you want and you already have an aur account, I can add you as Co-Maintainer

[mattrighetti]

the package needs to be bumped when you create a release here

Yeah that's what I thought, do you know if that can be done directly in a GitHub action? I can add a job in the current release CI job if needed.

The current release process creates a bunch of binaries for different archs and the corresponding SHA

[mroetsc]

I guess it could be done utilising this action:
https://github.com/marketplace/actions/aur-update-and-publish

The arch wiki states:

Automation is a valuable tool for maintainers, but it can not replace manual intervention (e.g. projects can change license, add or remove dependencies, and other notable changes even for "minor" releases). Automated PKGBUILD updates are used at your own risk and any malfunctioning accounts and their packages may be removed without prior notice.

Overall, I would suggest not including this in the pipeline. Manual bumping just offers more control and I do not think that it is that critical if the AUR package is outdated for a few days, at most. The aur also offers ways for end users to notify package maintainers about the package being outdated, which would result in notification via email for example.
So I would be in favor of updating the aur package manually.