Add license to plugin

[johnDeSilencio]

As of this evening, when I tried to build my Neovim config with this plugin using Nixvim, I got an error because this plugin has no license:

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

       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! :)