add edgeview container/api doc

Signed-off-by: Naiming Shen <naiming@zededa.com>

docs/EDGEVIEW-CONTAINER-API.md

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+# Edge-View Container and APIs
+
+EVE provides the Edge-View mainly for device and application troubleshooting tasks. It is a system-level container similar to `newlogd`, `wwan`, etc.
+
+Edge-View as a service on EVE, it needs to receive/update user configurations from the controller; and it needs to send/update Edge-View running status to the controller.
+
+The software integration level of Edge-View is somewhere between the services like `newlogd` and `wwan`. `newlogd` is an integral part of the EVE, while `wwan` is a generic Linux service without any specific EVE related changes. The Edge-View container runs on the EVE device and has many tasks designed specifically for EVE device and application usage, but it can also run as a normal Linux docker container on any host; the same container is used for Edge-View client runs on the user's laptop. Thus design of Edge-View container tries to minimize the interaction with the rest of the EVE system while still being configured from the controller and sending status to the controller.
+
+This Wiki page describes the provisioning, security and status update for Edge-View in [Edge-View Doc](https://wiki.lfedge.org/display/EVE/Edge-View).
+
+## Edge-View Configuration
+
+Edge-View configuration is part of the EVE device configuration. It contains mainly a JWT token string and policies. When `zedagent` receives the configuration update, it will write/modify the configuration file: `/run/edgeview/edge-view-config`. The configuration prefix strings are defined in `github.com/lf-edge/eve/pkg/pillar/type/edgeviewtypes.go`.
+
+Edge-View container has a script running, constantly monitors the configuration file content. The script will run and pass the configuration items into the edge-view programs or shut down the edge-view programs.
+
+## Edge-View Info Status
+
+The status of Edge-View is defined as `EdgeviewStatus` type in `github.com/lf-edge/eve/pkg/pillar/type/edgeviewtypes.go`. When there is a status change, it will update and send the Edge-View status through the info message to the controller.
+
+For security reasons, the Edge-View container has all the volumes mounted in 'read-only' mode. But for the status update, it needs to have write access on the device. Since the EVE pub/sub infrastructure mostly read/write into the '/run/{module}' directories, the Edge-View container volume mounts the `/run/edgeview` as 'read-write' mode as an exception for publishing the Edge-View status. It publishes status in `/run/edgeview` (this is similar to the `newlogd` container publishing status into `/run/newlogd` directory).
+
+## Logging to Event
+
+Edge-View logging is similar to other containers. For any user command received by Edge-View, it will log the client endpoint(IP address/port), the command and its parameters. The log entry will also be tagged with object-type of `log-to-event`, and the controller can optionally process those log entries and generate them as device events or alerts.