[Docs] Add a note about supported C++ toolchains, NFC.

This should help clarify the minimum support for building CIRCT, as
well as help avoid rare issues that can occur when different
toolchains are used to build LLVM versus CIRCT.

docs/GettingStarted.md

@@ -22,6 +22,10 @@ These commands can be used to setup CIRCT project:
   instructions](https://mlir.llvm.org/getting_started/), including cmake and 
 ninja.
 
+*Note:* CIRCT is known to build with at least GCC 9.4 and Clang 13.0.1, but
+older versions may not be supported. It is recommended to use the same C++
+toolchain to compile both LLVM and CIRCT to avoid potential issues.
+
 If you plan to use the Python bindings, you should start by reading [the
  instructions](https://mlir.llvm.org/docs/Bindings/Python/#building) for building
 the MLIR Python bindings, which describe extra dependencies, CMake variables,