Update instructions

README.md

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+# Synth
+
+## Introduction
+
+Build files are generated for various C++ compilers using CMake. For VS Code, you want CMake Tools and a supported compiler.
+
+## Dependencies
+
+PortAudio and PortMidi are included as Git submodules that need to be fetched before building:
+
+```
+git submodule init
+git submodule update
+```
+