Add license to plugin

As of this evening, when I tried to build my Neovim config with this plugin using [Nixvim](https://github.com/nix-community/nixvim), I got an error because this plugin has no license:

Trying to build Nixvim config fails because `git-conflict.nvim` lacks a license

```none
error: Refusing to evaluate package 'vimplugin-git-conflict.nvim-2.1.0' in /nix/store/ps8fhbglachs62pf47aw3rzx8zfij8sp-source/pkgs/applications/editors/vim/plugins/overrides.nix:1574 because it has an unfree license (‘unfree’)
a) To temporarily allow unfree packages, you can use an environment variable
for a single invocation of the nix tools.

$ export NIXPKGS_ALLOW_UNFREE=1

Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
then pass `--impure` in order to allow use of environment variables.

b) For `nixos-rebuild` you can set
{ nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.

Alternatively you can configure a predicate to allow specific packages:
{ nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
"git-conflict.nvim"
];
}

c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
{ allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.
```

@akinsho when you get a chance, would you mind adding a license to this plugin? Thank you! :)