add readme

Author: Natalia Kazakova (DevExpress)

Readme.md

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+*Files to look at*:
+
+* [Default.aspx.cs](./CS/AspNetWebFormsDataFederation/Default.aspx.cs) (VB: [Default.aspx.vb](./VB/AspNetWebFormsDataFederation/Default.aspx.vb))
+
+# ASP.NET Web Forms Dashboard - How to Register a Federated Data Source
+
+This example registers the [DashboardFederationDataSource](https://docs.devexpress.com/Dashboard/DevExpress.DashboardCommon.DashboardFederationDataSource) from the following set of [data sources](https://docs.devexpress.com/Dashboard/116522):
+
+* [DashboardSqlDataSource](https://docs.devexpress.com/Dashboard/401437) (the SQLite database)
+* [DashboardExcelDataSource](https://docs.devexpress.com/Dashboard/401433)
+* [DashboardObjectDataSource](https://docs.devexpress.com/Dashboard/401435)
+* [DashboardJsonDataSource](https://docs.devexpress.com/Dashboard/401431)
+
+The federated data source is stored in the in-memory storage ([DataSourceInMemoryStorage](https://docs.devexpress.com/Dashboard/DevExpress.DashboardWeb.DataSourceInMemoryStorage)) and is available from the [Add Data Source](https://docs.devexpress.com/Dashboard/117456/web-dashboard/create-dashboards-on-the-web/providing-data/manage-data-sources) dialog. Note that when you add a federated data source to a dashboard, all data sources used in the federated data source are also added to the dashboard.
+
+![](web-data-federation.png)
+
+This example demonstrates the following query types you can use to create a data federation:
+
+* **Join**
+ 
+ Combines rows from two or more tables based on a column they share. The join type specifies records that have matching values in both tables.
+
+* **Union and UnionAll**
+
+ **Union** combines rows from two or more tables into a single data set and removes duplicate rows in merged tables. **UnionAll** operates in the same manner as **Union**, but duplicates rows from different tables when they contain the same data. You can create a union query for data sources if data types of their columns are [implicitly converted](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/types/casting-and-type-conversions#implicit-conversions). 
+ 
+* **Transformation**
+
+ If a data source contains a complex column (an object), you can transform its properties to display them as separate columns in a flattened view. If one of the data column is an array, you can unfold its values and display a new data row for every element of the array. When you unfold the column, you can flatten it and create a flattened view.
+
+## Documentation
+
+* [ASP.NET Web Forms Framework - Register a Federated Data Source](https://docs.devexpress.com/Dashboard/402458)
+* [Register Default Data Sources](https://docs.devexpress.com/Dashboard/116300)
+
+## Examples
+
+- [ASP.NET Core Dashboard - How to Register a Federated Data Source](https://github.com/DevExpress-Examples/aspnet-core-dashboard-data-federation)
+- [ASP.NET MVC Dashboard - How to Register a Federated Data Source](https://github.com/DevExpress-Examples/aspnet-mvc-dashboard-data-federation)