Grammar installation instructions for Windows

[non-Jedi]

treesit-auto implicitly assumes the presence of a C compiler and linker on exec-path by relying on treesit-install-language-grammar for installing missing grammars. On linux this is a reasonable assumption, but it is rarely true on Windows. The grammar installation section of the README should either recommend either installing compiler tool-chain onto exec-path or directly downloading and manually installing the grammar. I'm not sure which is more preferable.