Add README to UHDM plugin

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>
diff --git a/uhdm-plugin/README.md b/uhdm-plugin/README.md
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+# UHDM Plugin
+
+The UHDM plugin has been renamed to [SystemVerilog](../systemverilog-plugin/).
+
+It is kept here for backwards compatibility reasons.
+When loaded, it shows a warning and loads the `SystemVerilog` plugin (if available).