docs: Add rationale of PureConfigModule

Author: avast-scala-builder

example/src/main/mdoc/pureconfig.md

@@ -8,6 +8,7 @@ This module allows you to load your application's configuration file according t
 [PureConfig](https://pureconfig.github.io) library to do so which uses [Lightbend Config](https://github.com/lightbend/config) which means
 that your application's configuration will be in [HOCON](https://github.com/lightbend/config/blob/master/HOCON.md) format.
 
+Loading of configuration is side-effectful so it is wrapped in `F` which is `Sync`. This module also tweaks the error messages a little.
 
 ```scala mdoc
 import com.avast.server.toolkit.pureconfig._
@@ -21,4 +22,4 @@ final case class ServerConfiguration(listenAddress: String, listenPort: Int)
 implicit val serverConfigurationReader: ConfigReader[ServerConfiguration] = deriveReader
 
 val maybeConfiguration = PureConfigModule.make[Task, ServerConfiguration]
-```
+```