examples/pmod: add nmigen pmod example

[TomKeddie]

As per #109

[ktemkin]

Example code in general is always good, but I'm not sure this is the right place for this.

Questions:

• Does this belong in the LUNA repository itself? This more seems like an example of using nmigen, rather than an example of anything LUNA-specific. (See the hw_features for LUNA-specific examples.)
• Is this a good example of PMOD use, if we do decide yes on the former? I'd prefer examples to be relatively self-contained and to have walkthrough-level documentation (like they would if they were in a minimal tutorial).

[TomKeddie]

@ktemkin thanks, I agree but I do think an example showing the pmod connectors in use would be good in luna. Perhaps an extension of blink that puts the led output through a cable looping pmod A to pmod B?

Alternately I'd be happy to add documentation. As per #72 the examples need a structure within which to document them.

[straithe]

Hi, Tom. Thank you for your PR! I'm working on updating the LUNA documentation so there is space for this example. It might be a while yet. I'll ping you as soon as a structure is in place for examples.

[TomKeddie]

@straithe cool, I'm still around. I passed my luna on to some who had a more immediate need but I'm fine with getting this example into shape as needed. Thanks for your hard work.

[miek]

I'm gonna go ahead and close this as we're moving board-specific components out of this repo. We'll integrate something like this into an example or tutorial as we flesh out the Cynthion documentation. Thanks!