Add readme

README.md

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+# ExprCpp
+Same as `std.Expr()`, but takes arbitrary C++ functions instead of MaskTools RPN expressions.
+
+## Example
+```
+user_func = '''
+int func(int x, int y) {
+    return x/4 + y/4;
+}
+'''
+clip00 = core.endill.expr_cpp(clip00, clip00, user_func)
+```
+
+## Building Prerequsites for Linux
+* Compiler with C++17 support
+* CMake 3.16.3+
+* LLVM 10 (`llvm-10-dev`)
+* Clang 10 (`libclang-cpp10-dev`)
+```
+mkdir build
+cd build
+cmake ..
+cmake --build .
+```
+
+## Building for Windows
+Only as a part of LLVM build, because Windows distibution of LLVM doesn't expose C++ interface.