document the duvet usage in h3

[Ruben2424]

Hi,
i have documented the duvet usage.
I hope that all i wrote is correct.

[eagr]

duvet is still under active development and the interface may change soon, as you could see here in the duvet-specification. Maybe we could keep this open and wait until the duvet 0.2 is out and link to the reference instead.

Also, it may be better to go into some Contribute section, along with a reminder of updating the annotations when behavior has changed.

[seanmonstar]

@eagr it's been a couple months, does it still seem like something that is going to change soon, or should we just merge Ruben's work here?

[eagr]

duvet has released a 0.2 but sadly the docs haven't landed, so we can't link to it yet. And the documentation pr seems a bit stagnant. Let's just do this.

Mind moving the docs into a CONTRIBUTING.md and refer to it from the README so that it can be more discoverable and less likely to be skipped by people? @Ruben2424

[Ruben2424]

@eagr I renamed it.