Update README.md

Author: egor-bogomolov

README.md

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-# `PSIMiner`
+# `[archived] PSIMiner`
 
 [![JetBrains Research](https://jb.gg/badges/research.svg)](https://confluence.jetbrains.com/display/ALL/JetBrains+on+GitHub)
 
+**Repository is archived as it requires a major rework following the updates to the IntelliJ Platform.**
+
 `PSIMiner` — a tool for processing PSI trees from the IntelliJ Platform.
 PSI trees contain code syntax trees as well as functions to work with them,
 and therefore can be used to enrich code representation using static analysis algorithms of modern IDEs.