More descriptive README

[Yann Hamdaoui]

The README is minimal. It's ok to have a small one for a start, but I think it lacks a basic description of what is in this repo (there's a shell.nix, there's node stuff, there's C stuff, there's Rust stuff), how can I use it, how can I build it, and also maybe how to run tests.

Something like:

This repository contains a tree-sitter grammar definition for the Nickel language, composed of the JSON files [...]. This definition can be fed directly to any code editor with tree-sitter support (please refer to the documentation of your editor to know how to do the integration).
The C parser generated by tree-sitter is available in src/parser.c, src/scanner.cc and src/tree_sitter (how to build it?).
Finally, this repository also contains Rust binding to this parser. The Cargo.toml file makes it a crate that can be used directly as a dependency by any Rust project, thanks to the [XXX][rust-tree-sitter-bindings] crate.
## Tests

To run the tests, do: run some tests.

I guess there's also something to be said about nodejs and the nix shell.