README missing tree-sitter-cli as a prerequisite

[MARLENELASSEINPHOTOMACHEN]

Problem

tree-sitter-cli is now a hard requirement of nvim-treesitter's main branch, which kickstart.nvim uses - without it, parser installation silently hangs on "Compiling parser" with no error message on every nvim launch.

Adding it to the README's external requirements was mentioned in #1657 but probably forgotten.

Expected

tree-sitter-cli should be listed alongside the other external requirements ripgrep and fd-find.

Installation instructions are available at: https://github.com/tree-sitter/tree-sitter/blob/master/crates/cli/README.md

#1862 appears to address this