add docs in introduction plugin

introduction/src/main/kotlin/IntroductionPlugin.kt

@@ -13,19 +13,40 @@ import org.jetbrains.exposed.sql.Database
 import org.koin.core.Koin
 import org.koin.core.module.Module
 
+/**
+ * This plugin represents simple plugin which realize all necessary functionality of plugin you may need in context of simple plugins.
+ * In context of this plugin we will decode plugin configuration and setup reaction on `/start` command. In `introduction`
+ * section of config you should put field `onStartCommandMessage` which will be sent to use on `/start` command
+ */
 @Serializable
 class IntroductionPlugin : Plugin {
+    /**
+     * Default logger of [IntroductionPlugin] got with [logger]
+     */
     private val log = logger
 
+    /**
+     * Configuration class for current plugin
+     *
+     * See realization of [setupDI] to get know how this class will be deserialized from global config
+     *
+     * See realization of [setupBotPlugin] to get know how to get access to this class
+     */
     @Serializable
     private class Config(
         val onStartCommandMessage: String
     )
 
+    /**
+     * DI configuration of current plugin. Here we are decoding [Config] and put it into [Module] receiver
+     */
     override fun Module.setupDI(database: Database, params: JsonObject) {
         single { get<Json>().decodeFromJsonElement(Config.serializer(), params["introduction"] ?: return@single null) }
     }
 
+    /**
+     * Final configuration of bot. Here we are getting [Config] from [koin] and configure reaction on `/start` command
+     */
     override suspend fun BehaviourContext.setupBotPlugin(koin: Koin) {
         val config = koin.getOrNull<Config>()