Add documentation regarding Custom preview annotation usage 🤩 📜 (#307)

* Add documentation regarding Custom preview annotation usage

Her I have added documentation for using Custom preview annotation with Showkase. I have added use cases and KSP vs KAPT differences. Also added docs
about how to use this with KAPT.

* Moved this documentation to an alternate section README.md

---------

Co-authored-by: Vinay Gaba <vinaygaba@gmail.com>

Author: oas004

README.md

@@ -386,6 +386,80 @@ val typography = metadata.typographyList
 
 ```
 
+##### 6. Custom/Multipreview Annotations
+You can use custom preview annotations for Showkase as well. Custom preview annotations are annotations that are annotated with preview it self.
+An example of this can be:
+
+```kt
+@Preview(name = "Custom Preview One First", group = "Custom Previews")
+@Preview(name = "Custom Preview One Second", group = "Custom Previews")
+annotation class CustomPreview1
+```
+
+```kt
+@Preview(name = "Custom Preview One First", group = "Custom Previews")
+annotation class CustomPreview2
+```
+
+this can be used to annotate a composable function like you would do any other preview function in Compose like this:
+
+```kt
+@CustomPreview1
+@Composable
+fun CustomAnnotationPreview() {
+}
+```
+
+This can also be combined like this:
+
+```kt
+@Preview(name = "Custom Preview One First", group = "Custom Previews")
+@Preview(name = "Custom Preview One Second", group = "Custom Previews")
+annotation class CustomPreviewOne
+
+
+@Preview(name = "Custom Preview Two First", group = "Custom Previews")
+annotation class CustomPreviewTwo
+
+@CustomPreviewOne
+@CustomPreviewTwo
+@Composable
+fun CustomAnnotationPreviewCombined() {
+}
+```
+
+###### KAPT vs KSP
+
+Because of [this issue](https://youtrack.jetbrains.com/issue/KT-49682/Support-JVM-IR-in-KAPT-stub-generation) KAPT does not quite support repeatable annotations from Kotlin. This means that  if you have an annotation class like:
+
+```kt
+@Preview(name = "Shape 100 by 100", group = "Shapes", widthDp = 100, heightDp = 100)
+@Preview(name = "Shape 150 by 150", group = "Shapes", widthDp = 150, heightDp = 150)
+annotation class CustomShape
+```
+
+This will be skipped by KAPT, but KSP will pick it up. However, if you have an annotation like:
+
+```kt
+@Preview(name = "Shape 100 by 100", group = "Shapes", widthDp = 100, heightDp = 100)
+annotation class CustomShape
+```
+It will be picked up by both KSP and KAPT.
+
+###### Important for KAPT users
+
+You will need to provide a compiler arg in you module for the custom preview annotations that you are using and expecting to be picked up by Showkase. This can be done with the following code:
+
+```kt
+kapt {
+    arguments {
+        arg("multiPreviewType", "com.airbnb.android.submodule.showkasesample.LocalePreview")
+    }
+}
+```
+
+It is important to remember to use the whole qualified name of the annotation, and not just the name.
+
 ## R8 / ProGuard
 
 If you use Showkase as a dependency in an Android project which uses R8 as a default compiler