introduce immutable-adapters in readme.md (#615)

README.md

@@ -5,6 +5,7 @@ Extensions for [Moshi](https://github.com/square/moshi)
 * [moshi-ir](https://github.com/ZacSweers/MoshiX/tree/main/moshi-ir) - A Kotlin IR implementation of Moshi and
   moshi-sealed code gen.
 * [moshi-adapters](https://github.com/ZacSweers/MoshiX/tree/main/moshi-adapters) - A collection of custom adapters for Moshi.
+* [mosh-immutable-adapters](https://github.com/ZacSweers/MoshiX/tree/main/moshi-immutable-adapters) - A collection of Moshi adapters for [kotlinx.collections.immutable](https://github.com/Kotlin/kotlinx.collections.immutable).
 * [moshi-metadata-reflect](https://github.com/ZacSweers/MoshiX/tree/main/moshi-metadata-reflect) - A [kotlinx-metadata](https://github.com/JetBrains/kotlin/tree/master/libraries/kotlinx-metadata/jvm) based implementation of `KotlinJsonAdapterFactory`. This allows for reflective Moshi serialization on Kotlin classes without the cost of including kotlin-reflect.
 * [moshi-sealed](https://github.com/ZacSweers/MoshiX/tree/main/moshi-sealed) - Reflective and code gen implementations for serializing Kotlin sealed classes via Moshi polymorphic adapters.