Add end date

[xfq]

Plan for a two-year charter, ending 31 March 2020, as proposed by @samuelweiler.

[anssiko]

I'd be fine with this, however, I'd like to note it'd be reasonable to expect one or more of the Device-related specifications currently in incubation (Web Bluetooth #28, Web USB #30, Web NFC #29) to be ready to advance to standards track sooner than 2020, and a possible landing zone for all of them would be this WG.

Web Bluetooth seems to be the most advanced, and likely the first candidate to make that transition.

Is there something we could do in this charter update to minimize the cost of mid-term rechartering required to bring in new work?

[anssiko]

I think the existing "Adding new Recommendation-track deliverables" section actually addresses my concern re mid-term scope expansion adequately.