Move factory flashing instructions and add basic example to README

[russss]

At the risk of me becoming the docs maintainer, this PR:

• Moves the factory flashing instructions out of the README and into docs/, as most users won't need this
• Adds some basic smoke-test commands to the README, so people have an example to test that everything's working

[attie]

As you're on this, please could you add a note re: glasgow flash, and it's importance for Windows users?

#206 (comment)

[russss]

I've added a warning about that to the flashing docs (it doesn't render especially well on GitHub but it looks nice in Sphinx).

[whitequark]

So there are two main issues with PR:

• Factory flashing isn't really "internal", it's a public interface that must be provided because Glasgow is OSHW and a lot of people are ordering boards for themselves. That will probably be even more if/when we ship Edinburgh, optimized for low-cost fabs.
• "Factory flashing" and "flashing" are not the same thing and the WinUSB warning applies to the non-flashed boards, not non-factory-flashed boards. The former term probably needs to be changed.

I'm not sure if it makes more sense for you to fix the PR or for me to do it myself, since the review overhead here would be higher than the actual effort to fix the docs.

[russss]

I'm happy for you to edit.

I still think that the number of people who need to factory flash is going to be small compared to those who have a flashed device and want to get going with it, so it makes sense to put those instructions somewhere other than the README, notwithstanding whether it's classified as "internal" or not.

[whitequark]

We now have proper documentation so this PR is no longer relevant. Thank you for the work you've done, @russss!