Add distribution specific install hints

[tomschr]

This PR fixes #196

• Improve the installation section and add installation instructions for Arch Linux, Debian, Fedora, FreeBSD, openSUSE, and Ubuntu.
• Adapt CHANGELOG.rst

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I've mostly used the installation instructions from https://pkgs.org, but I'm not sure if this is all correct (I mainly use openSUSE). So please check, if this is ok.

[ppkt]

Looks great, thank you :)
I would add information that in some distros (ekhem, Debian) semver package is outdated and won't have latest bugfixes or features :)

[tomschr]

Thanks Karol for your feedback!

Looks great, thank you :)

Thanks. 😊

I would add information that in some distros (ekhem, Debian) semver package is outdated and won't have latest bugfixes or features :)

Yes, that's also what I've noticed. As this affects all distributions, it has to be a general note. Maybe something like this:

You will notice that some Linux distributions can have outdated packages.
These outdated packages does not contain the latest bug fixes or new features.
If you need a newer package, you have these option:

* Ask the packager to update the package.
* Update the package for your favorite distribution and submit it.
* Use a virtual environment and `pip install`.

I'm a bit still unsure about the wording, but this is the idea.

[ppkt]

@tomschr yes, it's very clear now :)
In meantime I'll check who is responsible for semver in Ubuntu / Debian and I'll ask them about possible update.

[scls19fr]

Thanks guys for your involvment in this project!