Update README.md

[diyelectromusic]

Added recommendation to use an older or cheaper Pi (not a Pi 5) for a first build.

[probonopd]

Thanks @diyelectromusic. Merged with slight wording changes.

[diyelectromusic]

Well I was trying to get over that given the experimental nature, and uncertainty about the cooling, it is a lot less risk to do a build with a cheaper Pi, and as there is no performance advantage to speak off, it is very much recommended.

Now it reads like "well, there isn't really a reason to use the Pi 5 at the moment, but see how you go" but I really wanted to say "look seriously, at this stage, I recommend you don't use a Pi 5 unless you know what you are doing and are prepared to take the risk"... hence the more commanding nature and the bold highlighting...

But it's your project and if people break their shiny new Pi 5 then I guess they get to keep both pieces...

Kevin