Update README.md

deprecate (but not archive) core

README.md

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+## Deprecation Notice
+
+Although not yet archived, this repository is no longer maintained outside of patches to versions `4.2.x` of `cellProfiler-core`. For version 5+, it has been made a subpackage of the main [CellProfiler repo](https://github.com/CellProfiler/CellProfiler/tree/main/src/subpackages/core).
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 # CellProfiler-core
 
 CellProfiler-core contains the critical classes and components required by [CellProfiler](cellprofiler.org), whereas the [main repository](https://github.com/CellProfiler/CellProfiler) contains non-essential modules and the GUI interface. This package can be used to access CellProfiler's functionality as a Python package, see [this page](https://github.com/CellProfiler/CellProfiler/wiki/CellProfiler-as-a-Python-package) for more details.