update readme

README.md

@@ -13,6 +13,8 @@ I have written extensively about my kicad melting experiments [here](https://mit
 
 __For the KiCad 5 compatible version, please use the kicad-5 branch.__
 
+__KiCad Nightly (6.99) is not supported.__
+
 The intended use is to have a source PCB file, which is easy to edit, and a rendered output file, generated by this plugin. After running DRC on the output, you can then go back and adjust the original file before running the plugin again.
 
 If you select part of the board, only the selected tracks will get melted. If nothing is selected, the whole file will be processed.