systemverilog-plugin: Add description for simplify files in third_party/yosys

Signed-off-by: Magdalena Andrys <mandrys@antmicro.com>

systemverilog-plugin/third_party/yosys/README

diff --git a/systemverilog-plugin/third_party/yosys/README b/systemverilog-plugin/third_party/yosys/README
index ceb62a9..df69e69 100644
--- a/systemverilog-plugin/third_party/yosys/README
+++ b/systemverilog-plugin/third_party/yosys/README
@@ -11,7 +11,21 @@
     - Removed Yosys namespace; `const2ast()` has been placed inside
       `systemverilog_plugin` namespace to avoid conflicts with the symbol from
       Yosys when statically linking.
+- simplify.cc: yosys/frontends/ast/simplify.cc (rev. ceef00c)
+  - The file is a part of Yosys AST frontend. It has been placed in the plugin,
+    as in some cases we need to adjust it to support certain functionalities
+    in the plugin. Since it is included now in the plugin, we can skip caling
+    the original Yosys' simplify() during AST preparation. The original Yosys'
+    simplify() is only called in uhdmcommonfrontend.cc when Yosys' process()
+    is called, after having AST done.
+  - Changes:
+    - Removed unneeded code and member functions of AstNode::
+    - Modified usage of AstNode:: members that are called from the Yosys'
+      AstNode:: struct.
+  - The file will be extended in the future instead of simplify_sv()
+    in UhdmAst.cc, and it will be moved to other directory then.
 
 Non-copied files placed here for interfacing purposes:
 
 - const2ast.h
+- simplify.h