Add introduction and key points

[robatwilliams]

See spec & explainer from WICG. Short intro, don't explain the whole thing.

• why it's different from WebUSB, Web Bluetooth
• it's an abstraction (events, input/output/feature reports) over lower-level read/write APIs, usage pages
• protected usages - keyboard, ...
• Cannot interact with non-GUI HID interface WebBluetoothCG/web-bluetooth#393

Anything else from discussions?

• https://discourse.wicg.io/t/human-interface-device-hid-api/3070
• https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/OaDCpCaEe_4/3taK3m75DAAJ

Limitations on Windows platform?

• https://developers.google.com/web/fundamentals/native-hardware/build-for-webusb/
• Windows not recognizing devices in WebUSB WICG/webusb#143

[robatwilliams]

I don't believe (although I'm not sure) that the driver capture issues that Windows has with WebUSB will apply to WebHID.

Other links on the subject:

• https://developers.google.com/web/fundamentals/native-hardware/build-for-webusb/
• Windows not recognizing devices in WebUSB WICG/webusb#143
• https://stackoverflow.com/questions/46179569/smartcard-reader-access-denied-while-claiming-interface-with-webusb-on-chrome
• https://medium.com/@gendor/connecting-to-usb-devices-with-your-browser-d433a6df6f2

[robatwilliams]

23a5a36 short intro
0737d03 key points