init research

dataframe/docs/StardustDocs/topics/addDf.md

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+[//]: # (title: add)
+
+<!---IMPORT org.jetbrains.kotlinx.dataframe.samples.api.Modify-->
+
+Returns [`DataFrame`](DataFrame.md) with union of columns from several given [`DataFrame`](DataFrame.md) objects.
+
+<!---FUN addDataFrames-->
+
+```kotlin
+df.addAll(df1, df2)
+```
+
+<inline-frame src="resources/org.jetbrains.kotlinx.dataframe.samples.api.Modify.addDataFrames.html" width="100%"/>
+<!---END-->
+
+See [all use cases of 'add' operation](add.md).