update core readme

Author: AndreiKingsley

core/README.md

@@ -21,3 +21,27 @@ See the [KDoc Preprocessing Guide](../KDOC_PREPROCESSING.md) for more informatio
 KDocs can also be exported to HTML, for them to be reused on the website.
 Elements annotated with `@ExportAsHtml` will have their generated content be copied over to
 [docs/StardustDocs/resources/snippets/kdocs](../docs/StardustDocs/resources/snippets/kdocs).
+
+### ~~Korro~~ (NOTE: This is being moved to [:samples](../samples))
+
+> [Should be removed and moved to `:samples` module](https://github.com/Kotlin/dataframe/issues/898).
+
+Tests in this module contain code samples used for import into documentation 
+using [Korro](https://github.com/devcrocod/korro).
+
+
+### ~~Explainer dataframes~~ (NOTE: This is being moved to [:samples](../samples))
+
+> [Should be removed and migrated to SampleHelper](https://github.com/Kotlin/dataframe/issues/898).
+
+Aside from code samples, `@TransformDataFrameExpressions` annotated test functions also generate sample
+dataframe HTML files that can be used as iFrames on the documentation website.
+They are tested, generated, and copied over to [docs/StardustDocs/resources/snippets](../docs/StardustDocs/resources/snippets) by
+our "explainer" [plugin callback proxy](./src/test/kotlin/org/jetbrains/kotlinx/dataframe/explainer),
+which hooks into [the TestBase class](./src/test/kotlin/org/jetbrains/kotlinx/dataframe/samples/api/TestBase.kt) and
+retrieves the intermediate DataFrame expressions thanks to our "explainer" compiler plugin
+[:plugins:expressions-converter](../plugins/expressions-converter).
+
+We can also generate "normal" DataFrame samples for the website. This can be done using the
+[OtherSamples class](./src/test/kotlin/org/jetbrains/kotlinx/dataframe/samples/api/OtherSamples.kt). Generated
+HTML files will be stored in [docs/StardustDocs/resources/snippets/manual](../docs/StardustDocs/resources/snippets/manual).