README.md: Warn about buggy snap packages

[Enselic]

As discussed in #1519, the batcat snap package is too problematic for
official endorsement, so withdraw recommendation from README.me (and
remove section from CHANGELOG.md since a release with it has not been
made yet).

For now this MR should just be seen as a sketch of how we could go about doing it, in case we end up choosing this path.

[eth-p]

I think calling the existing packages "buggy" can come off as rude or callous.

It might be better to say "Existing packages may be available, but are not officially supported and may contain issues."

With the link on the word "issues".

[sharkdp]

Thanks!

[sharkdp]

Unfortuntely, Ubuntu shows this if someone tries to install batcat (instead of bat, which is still the package name)

$ sudo apt install batcat
Reading package lists... Done
Building dependency tree       
Reading state information... Done

No apt package "batcat", but there is a snap with that name.
Try "snap install batcat"