[camera_web] Update the web plugin README (flutter#4237)

* docs: update web plugin README
* docs: update web plugin missing implementation README

Author: bselwe

packages/camera/camera_web/README.md

@@ -1,7 +1,77 @@
 # Camera Web Plugin
 
-A Flutter plugin for Web allowing access to the device cameras.
+The web implementation of [`camera`][camera].
 
-*Note*: This plugin is under development.
+*Note*: This plugin is under development. See [missing implementation](#missing-implementation).
 
-In order to use this plugin, your app should depend both on `camera` and `camera_web`. This is a temporary solution until a plugin is released.
+## Usage
+
+This package is [endorsed](https://flutter.dev/docs/development/packages-and-plugins/developing-packages#endorsed-federated-plugin), which means you can simply use `camera` normally. This package will be automatically included in your app when you do.
+
+## Example
+
+Find the example in the [`camera` package](https://pub.dev/packages/camera#example).
+
+## Limitations on the web platform
+
+### Camera devices
+
+The camera devices are accessed with [Stream Web API](https://developer.mozilla.org/en-US/docs/Web/API/Media_Streams_API) with the following [browser support](https://caniuse.com/stream):
+
+![Data on support for the Stream feature across the major browsers from caniuse.com](https://caniuse.bitsofco.de/image/stream.png)
+
+Accessing camera devices requires a [secure browsing context](https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts). This means that you might need to serve your web application over HTTPS. For insecure contexts `CameraPlatform.availableCameras` might throw a `CameraException` with the `permissionDenied` error code.
+
+### Device orientation
+
+The device orientation implementation is backed by [`Screen Orientation Web API`](https://www.w3.org/TR/screen-orientation/) with the following [browser support](https://caniuse.com/screen-orientation):
+
+![Data on support for the Screen Orientation feature across the major browsers from caniuse.com](https://caniuse.bitsofco.de/image/screen-orientation.png)
+
+For the browsers that do not support the device orientation:
+- `CameraPlatform.onDeviceOrientationChanged` returns an empty stream.
+- `CameraPlatform.lockCaptureOrientation` and `CameraPlatform.unlockCaptureOrientation` throw a `PlatformException` with the `orientationNotSupported` error code.
+
+### Flash mode and zoom level
+
+The flash mode and zoom level implementation is backed by [Image Capture Web API](https://w3c.github.io/mediacapture-image/) with the following [browser support](https://caniuse.com/mdn-api_imagecapture) (as of 12 August 2021):
+
+![Data on support for the Image Capture feature across the major browsers from caniuse.com](https://caniuse.bitsofco.de/static/v1/mdn-api__ImageCapture-1628778966589.png) 
+
+For the browsers that do not support the flash mode:
+- `CameraPlatform.setFlashMode` throws a `PlatformException` with the `torchModeNotSupported` error code.
+
+For the browsers that do not support the zoom level:
+- `CameraPlatform.getMaxZoomLevel`, `CameraPlatform.getMinZoomLevel` and `CameraPlatform.setZoomLevel` throw a `PlatformException` with the `zoomLevelNotSupported` error code.
+
+### Taking a picture
+
+The image capturing implementation is backed by [`URL.createObjectUrl` Web API](https://developer.mozilla.org/en-US/docs/Web/API/URL/createObjectURL) with the following [browser support](https://caniuse.com/bloburls):
+
+![Data on support for the Blob URLs feature across the major browsers from caniuse.com](https://caniuse.bitsofco.de/image/bloburls.png)
+
+The web platform does not support `dart:io`. Attempts to display a captured image using `Image.file` will throw an error. The capture image contains a network-accessible URL pointing to a location within the browser and should be displayed using `Image.network` or `Image.memory` after loading the image bytes to memory.
+
+See the example below:
+
+```dart
+if (kIsWeb) {
+  Image.network(capturedImage.path);
+} else {
+  Image.file(File(capturedImage.path));
+}
+```
+
+## Missing implementation
+
+The web implementation of [`camera`][camera] is missing the following features:
+- Video recording
+- Exposure mode, point and offset
+- Focus mode and point
+- Camera closing events
+- Camera sensor orientation
+- Camera image format group
+- Camera image streaming
+
+<!-- Links -->
+[camera]: https://pub.dev/packages/camera