Add README.md for Web NFC.

This CL adds some documentation for the NFC module in Chrome.
The detailed design documentation is located at
https://sites.google.com/a/chromium.org/dev/developers/design-documents/web-nfc
and the READMEs do reference that document for more details.

Bug: 520391
Change-Id: Ib8b7632d0bf9bba2b3de9fb1b9bf4f528fa11162
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1985961
Commit-Queue: Rijubrata Bhaumik <rijubrata.bhaumik@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Leon Han <leon.han@intel.com>
Reviewed-by: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#729664}

services/device/nfc/README.md

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+# NFC
+
+### High level Overview
+
+The implementation of Web NFC in Chromium consists of two main parts:
+The NFC module in Blink located at `third_party/blink/renderer/modules/nfc/` which
+contains Blink JavaScript bindings for Web NFC and the browser side platform
+level adaptation that is located at `services/device/nfc`. The Blink NFC module
+communicates with the browser adaptation through NFC Mojo interface defined in
+the `services/device/public/mojom/nfc.mojom` file and implemented by the
+`services/device/nfc` module.
+
+NDEFWriter and NDEFReader are the two primary interfaces of the Web NFC APIs.
+
+The NDEFWriter interface has the push method for writing data to NFC devices such as
+tags. This method will return a promise, which will be resolved when the
+message is successfully written to some target or be rejected when errors
+happened or the process is aborted by setting the AbortSignal in the
+NDEFPushOptions.
+
+The NDEFReader interface has the scan method to try to read data from any NFC device
+that comes within proximity. Once there is some data found to be matching the
+filtering criteria provided by web developers in NDEFScanOptions, an
+NDEFReadingEvent carrying the data will be dispatched to the NDEFReader.
+
+The most important classes for Android adaptation are
+
+[NfcImpl](../../../services/device/nfc/android/java/src/org/chromium/device/nfc/NfcImpl.java),
+[NfcTagHandler](../../../services/device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java)
+and
+[NdefMessageUtils](../../../services/device/nfc/android/java/src/org/chromium/device/nfc/NdefMessageUtils.java).
+
+## Web-exposed Interfaces
+
+### [NFC specification ](https://w3c.github.io/web-nfc/)
+
+## Testing:
+
+* Web platform tests are located in
+`third_party/blink/web_tests/external/wpt/web-nfc/` and are a mirror of the
+[web-platform-tests GitHub repository](https://github.com/web-platform-tests/wpt).
+* NFC platform unit tests file for Android is
+[NFCTest.java](../../../services/device/nfc/android/junit/src/org/chromium/device/nfc/NFCTest.java)
+
+
+## Security and Privacy
+
+Web NFC API can be only accessed by top-level secure browsing contexts and user
+permission is required to access NFC functionality. Web NFC API specification
+addresses security and privacy topics in chapter [7. Security and Privacy](https://w3c.github.io/web-nfc/#security).
+
+
+## Permissions
+
+The device service provides no support for permission checks. When the render
+process requests access to a NFC this request is proxied through the browser
+process by [NfcPermissionContext](../../../chrome/browser/nfc/nfc_permission_context.h)
+which is responsible for checking the permissions granted to the requesting origin.
+
+
+## Platform Support
+
+At the time of writing, only Android platform is supported.
+
+
+## Design Documents
+
+Please refer to the [design documentation](https://sites.google.com/a/chromium.org/dev/developers/design-documents/web-nfc)
+for more details.

third_party/blink/renderer/modules/nfc/README.md

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+# Web NFC Blink Module
+
+`renderer/modules/nfc` implements the renderer process details and bindings for
+the [Web NFC specification]. It communicates with the browser process through the
+[Web NFC Mojo interface]. The platform-specific parts of the
+implementation are located in `services/device/nfc`.
+
+[Web NFC specification]: https://w3c.github.io/web-nfc/
+[Web NFC Mojo interface]: ../../../../../services/device/public/mojom/nfc.mojom
+
+
+## Testing
+
+Web NFC is primarily tested in [Web Platform Tests](https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/web_tests/external/wpt/web-nfc/).
+
+
+## Design Documents
+
+Please refer to the [design documentation](https://sites.google.com/a/chromium.org/dev/developers/design-documents/web-nfc)
+for more details.