Update README.md.

Author: Tweety-Lab

README.md

@@ -1,6 +1,18 @@
 # Binaryen.NET
 C# Bindings for the Binaryen WebAssembly toolchain.
 
+## Building
+- **If you only changed C# code:** The project automatically uses the pre-built native binaries from `runtimes/`, no manual compilation is needed.
+- **If you updated Binaryen itself:** Rebuild Binaryen as a shared library, then place the compiled binaries for each target runtime into the `runtimes/` folder.
+
+You can build the C# project by running:
+```bash
+dotnet build
+```
+from the directory containing `Binaryen.NET.sln`.
+
+To build or update the native Binaryen library (`libbinaryen`), follow the instructions in the [Binaryen repository](https://github.com/WebAssembly/binaryen).
+
 ## Samples
 
 ### Generating IR -> WAT