add branch overview

README.md

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+# Article Voucher
+
+- 4.x - OXID 6.1 to 6.5
+  - [for ionCube from PHP 7.1](https://github.com/d3datadevelopment/Articlevoucher/tree/rel_ionCube_PHP71-74)
+  - [for SourceGuardian from PHP 8.0](https://github.com/d3datadevelopment/Articlevoucher/tree/rel_4.x_sourceGuardian_PHP8.0)
+- 5.x - OXID 7.x
+  - [for SourceGuardian from PHP 8.0](https://github.com/d3datadevelopment/Articlevoucher/tree/rel_5.x_sourceGuardian_PHP8.0)
+
+** main branch left blank **