Update readme

Author: tealeg

README.org

@@ -1,3 +1,10 @@
+* NOTICE
+*Effective immediately*: XLSX has been migrated to [[https://codeberg.org/tealeg/xlsx][codeberg]] and it's version bumped to v4.
+
+No further support or maintenance will be provided on the GitHub
+hosted version of this repository.  Please migrate to codeberg if you
+wish to contribute to or use this library.
+
 * XLSX
 
 [[https://github.com/tealeg/xlsx/actions/workflows/go.yml][https://github.com/tealeg/xlsx/actions/workflows/go.yml/badge.svg?branch=master]]
@@ -78,6 +85,9 @@ be held in whilst processing.
 
 StreamFileBuilder has been dropped from this version of the library as it has become difficult to maintain. 
 
+*** Version 4.x.x
+Migrated to https://codeberg.org/tealeg/xlsx 
+
 ** Full API docs
 The full API docs can be viewed using go's built in documentation
 tool, or online at [[https://pkg.go.dev/github.com/tealeg/xlsx/v3][pkg.go.dev]].