add docs

plugin/src/main/kotlin/dev/inmo/plagubot/Plugin.kt

@@ -6,8 +6,22 @@ import dev.inmo.tgbotapi.updateshandlers.FlowsUpdatesFilter
 import kotlinx.coroutines.CoroutineScope
 import org.jetbrains.exposed.sql.Database
 
+/**
+ * Use this interface for your bot. It is possible to use [kotlinx.serialization.SerialName] annotations on your plugins
+ * to set up short name for your plugin. Besides, simple name of your class will be used as key for deserialization
+ * too.
+ */
 interface Plugin {
+    /**
+     * In case you want to publish some processed by your plugin commands, you can provide it via this method
+     *
+     * @see BotCommand
+     */
     suspend fun getCommands(): List<BotCommand> = emptyList()
+
+    /**
+     * This method (usually) will be invoked just one time in the whole application.
+     */
     suspend operator fun invoke(
         bot: TelegramBot,
         database: Database,