Create Function Registration.md

Author: govert

docs/guides-basic/Function Registration.md

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+---
+title: "Function Registration"
+---
+**Note:** This document reflects changes made in Excel-DNA v1.9.
+
+Excel-DNA support the creation of user-defined functions for Excel in .NET. This document describes how functions are selected for registration, supported method signatures, method conversions and extension points.
+
+
+
+
+### Changes from earlier versions
+In v1.9 we incorporate the functionality previously exposes in the separate `ExcelDna.Registration` library (and package) into the main Excel-DNA library.
+To use the extended registration features under older versions, explict registration was required.
+Under v1.9 we have:
+* expanded the supported parameter and return types for functions markes with `[ExcelFunction]`.
+* added support for async and streaming functions and object handles (with the `[return:ExcelHandle]` and `[ExcelHandle]` attributes) in the main library
+* migrated registration extension points like `FunctionExecutionHandler` from the ExcelDna.Registration package to the main ExcelDna.Integration library.