message: Add mentions of requirements before the build instructions
  The `Getting Started` instructions requires git, ninja, and a working C++ host toolchain.
diff: @@ -84,7 +84,11 @@ and by making references explicit with `function_ref`.
  # Getting started with MLIR
  -MLIR has been tested on Linux and macOS, with recent versions of clang and with gcc 7.
  +The following instructions assume that you have `git`,
  +[`ninja`](https://ninja-build.org/), and a working C++ toolchain. In the future,
  +we aim to align on the same level of platform support as
  +[LLVM](https://llvm.org/docs/GettingStarted.html#requirements). For now, MLIR
  +has been tested on Linux and macOS, with recent versions of clang and with gcc 7.
  ```
  git clone https://github.com/llvm/llvm-project.git
  