Added a minimal README file for the Video Capture Service.

Format is based on services/device/generic_sensor and services/device/usb.

Change-Id: Ib21174e9b30ca9f53da90752ff3c423b31d9b57c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620739
Auto-Submit: Michael Martis <martis@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842950}

services/video_capture/README.md

+`services/video_capture` facilitates access to native video capture devices such as webcams and built-in cameras.
+
+See the [service design
+document](https://docs.google.com/document/d/1Qw7rw1AJy0QHXjha36jZNiEuxsxWslJ_X-zpOhijvI8) for more detailed information.