GPU compatibility with Linux-libre

[Bone Baboon]

This message has two parts about GPU compatibility with Linux-libre.
1) Manual Warning
2) Summary

# Manual Warning

I originally posted the message below on the help mailing list.  It is
reposted here for two reasons.

* It was suggested that the help mailing list may not be the correct
  mailing list. 
  https://lists.gnu.org/archive/html/help-guix/2021-04/msg00229.html

* Someone else recently had the same issue with GPU compatibility.
  https://issues.guix.gnu.org/issue/48343

>From the original message:

```
I found the Hardware Considerations section of the Guix manual very
helpful.  I was prepared for the issue of WiFi device compatibility.
However I was surprised by GPU compatibility.

I think it would be helpful to have a warning about GPU compatibility.
This would help someone who is looking at buying a computer by bringing
GPU compatibility to their attention.

I found the WiFi device warning with specific hardware that is known to
work very helpful.  Maybe an equivalent could be done for GPUs

...

I appreciate that this extra warning may discourage some people from
trying Guix.  I think it would still be good to warn people about
potential problems so that they are not surprised.
```

# Summary

I have been sharing a link to "GPU compatibility with Linux-libre"
https://lists.gnu.org/archive/html/help-guix/2021-04/msg00220.html on
the Guix mailing lists and in #guix when people were asking for help
about getting a graphical environment setup in Guix. The feedback I have
received is that it was helpful.  I would appreciate any correction or
additions as I do not want to misinform people.