Aside: we need to start adding more columns to say what's on SAMD51, nRF52, etc.

@dhalbert I'll work on adding the others. I have a few precursory questions:

1. Will there be, or are there, Express versions of the SAMD51?

2. For the nRF boards, should we break them out like SAMD (i.e. nRF51, nRF52)?

3. Should we break up the table? On my screen resolution (1680x1050), it looks like there will be enough width to display a single table on GitHub. Viewing on RTD might be constrained even at my res. Looking at the page source I don't see any explicit sizes defined for the content <div>s, but they are definitely standard and don't extend the full width of the rendered page. With all that, I'm sure there are still people out there with sub 1600 resolutions, so need to think about that at minimum...

4. Do you want me to submit a new issue? Or will you just update issue Add audiobusio to the Support Matrix in the doc #448 and leave it open?

The Metro M4 prototype boards that people have are Express boards. But the M4 has much more RAM and flash, so the distinction is less important: an M4 even without SPI flash will probably be able to include all modules. The other categories are yet to be determined, so until 3.0 is closer we won't know.

The use of a full matrix will become less important, since the M4 boards will be big enough to handle everything. We might want to drop this as a complete Cartesian product table at some point, and instead say "supported on all boards", and then list particular omissions for particular boards.