Add deprecation warning to README.md

README.md

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+> **⚠ DEPRECATION WARNING**
+> 
+> The development of this qudi repository has ceased. We strongly recommend migrating to our new [qudi-core project](https://github.com/Ulm-IQO/qudi-core) that is actively developed. Framework functionality is now separated from the measurement toolchains. Most features of the old project can be recovered by installing the qudi-core and qudi-iqo-modules packages.
+
 # qudi
 Qudi is a suite of tools for operating multi-instrument and multi-computer laboratory experiments.
 Originally built around a confocal fluorescence microscope experiments, it has grown to be a generally applicable framework for controlling experiments.