Add manual installation commands for syntax highlighting in Neovim (#8038)

[Carbon Syntax Highlighting for Vim & Neovim][1] is missing manual
installation commands for Neovim.

Custom syntax files have to be present in `~/.config/nvim/syntax/`
([source][2]) and filetypes in `~/.config/nvim/ftdetect` ([source][3]).


[1]: https://github.com/carbon-language/carbon-lang/tree/trunk/utils/vim
[2]: https://neovim.io/doc/user/syntax/#mysyntaxfile
[3]: https://neovim.io/doc/user/filetype/#new-filetype

Co-authored-by: Christopher Di Bella <cjdb@google.com>

utils/vim/README.md

@@ -23,12 +23,19 @@ and submit pull requests to the main repository.
 
 ## Manual Installation
 
-### Vim Users
-
 From the current directory `utils/vim`, please run the following commands to
 install the syntax file.
 
+### Vim Users
+
 ```
 mkdir -p ~/.vim/syntax && cp syntax/carbon.vim ~/.vim/syntax/
 mkdir -p ~/.vim/ftdetect && cp ftdetect/carbon.vim ~/.vim/ftdetect/
 ```
+
+### Neovim Users
+
+```
+mkdir -p ~/.config/nvim/syntax && cp syntax/carbon.vim ~/.config/nvim/syntax/
+mkdir -p ~/.config/nvim/ftdetect && cp ftdetect/carbon.vim ~/.config/nvim/ftdetect/
+```