doc: add the logging guide

docs/Logging.md

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+# Logging
+
+To debug the application, it is useful to enable logging.
+
+By default, the plugin outputs INFO level logs.\
+You can change the log level at runtime as follows:
+
+```cs
+Mediapipe.Unity.Logger.MinLogLevel = Logger.LogLevel.Debug;
+```
+
+Additionally, MediaPipe uses [Google Logging Library(glog)](https://github.com/google/glog) internally.\
+You can configure it by [setting flags](https://github.com/google/glog#setting-flags).
+
+```cs
+Glog.Logtostderr = true; // when true, log will be output to `Editor.log` / `Player.log`
+Glog.Minloglevel = 0; // output INFO logs
+Glog.V = 3; // output more verbose logs
+```
+
+To enable those flags, call `Glog.Initialize` when the application starts.
+
+```cs
+Glog.Initialize("MediaPipeUnityPlugin");
+```
+
+> :skull_and_crossbones: `Glog.Initialize` can be called only once. The second call will crash your application or UnityEditor.
+
+> :bulb: If you look closely at `Editor.log`, you will notice the following warning log output.\
+>
+> ```txt
+> WARNING: Logging before InitGoogleLogging() is written to STDERR
+> ```
+>
+> To suppress it, you need to call `Glog.Initialize`.\
+> However, without setting `true` to `Glog.Logtostderr`, glog won't output to `Editor.log` / `Player.log`.

docs/README.md

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+# Contents
+
+- [Build](Build.md)
+- [Logging](Logging.md)
+
+## Tutorials
+
+- [Tutorial - Task API](Tutorial-Task-API.md)
+- [Tutorial - Hello World!](Tutorial-Hello-World.md)
+- [Tutorial - Custom Graph](Tutorial-Custom-Graph.md)
+
+## API Reference
+
+:construction: Under construction