Document supported platforms

[SeanTAllen]

We do prebuilts for

• MacOS x86
• Linux x86
• Windows x86

There are others such as FreeBSD, MacOS arm that might have old versions that are unsupported. Anyone not on one of the three supported platforms should be directed to build from source.

[SeanTAllen]

If you start from the ponyc repo this documented. But its not obvious if you start from ponyup repo.