Document gopass extension framework for custom commands

[chaorace]

This line in the README implies that custom commands should be possible via extensions.

gopass/README.md

Line 47 in d6acc70

| Extensions | | Extend gopass with custom commands using our API |

However, there is no further documentation either for creating extensions nor registering custom commands anywhere in the repository.

[dominikschulz]

Indeed. We need to explain how this works.

[chaorace]

Understood if this isn't a high priority at the moment.

Of course, I'd be very thankful if anyone knows of a good example extension I could reference. If not, perhaps just a link to gopass's extension loader code? All hints are appreciated ❤️

[dominikschulz]

This is probably the best example we have right now: https://github.com/gopasspw/gopass-hibp

It's maybe not the kind of "extension" you were expecting. I'll need to write something about the reasoning and how to extend gopass.