Update instructions for getting tar-files

[tomtrafford]

No description provided.

[Araneidae]

Please reference issue #62 as I think we need to discuss how to proceed in a bit more detail.

Unfortunately I think there are still several issues that need to be addressed:

• First of all, what do we (or other users) do when any of the tar file sources is unavailable? This does happen: links change, sites move, or even sometime sites go offline temporarily or even permanently. This is why I have always urged for a curated collection of definitive copies of the tar files (and this is why the MD5 sums are always checked).
• Secondly, the user needs to be advised that there are now two targets available (zynq and zynqmp, I think). These need to be mentioned in the documentation.
• Thirdly, the list of MD5 sums in the document is good ... but it's a headache to maintain, and I can see already that it is out of date, it doesn't match the list of files being downloaded by the referenced script.
• Finally, I see that we do generate a sensible looking collection of assets in the Releases page. How is this generated? This should definitely be referenced.

[Araneidae]

As noted this change doesn't really address the underlying problems. See #62 for discussion.