Add info about Neovim and Tree-sitter to README

necessary by appending `.jinja` to it.

Note for Neovim and Tree-sitter

===============================

If Neovim's Tree-sitter syntax highlighting is active regular syntax

highlighting will be disabled.  This means for example that if you edit a

`html.jinja` file and you have the Tree-sitter HTML parser installed the

regular highlighting for Jinja will be disabled.  If you want to have both

Tree-sitter highlighting for HTML and regular highlighting for Jinja in your

buffer you need to set `syntax=on` for that buffer.

One way of automating this is to add this snippet to `ftplugin/jinja.vim`:

.. code:: vim

   if !get(b:, 'jinja_syntax_autocmd_loaded', v:false)

	   if luaeval("vim.treesitter.language.get_lang('jinja')") == v:null

		   autocmd FileType <buffer> if !empty(&ft) | setlocal syntax=on | endif

	   endif

	   let b:jinja_syntax_autocmd_loaded = v:true

   endif

Bugs and Caveats

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