doc: Instruct to generate Gemfile.lock file in Nix

The lockfile is used for generating gemset.nix using bundix.

In Nix (but not in the Docker image), github-pages/jekyll fail with the
error below if the lockfile is missing, due to BUNDLE_FROZEN being set
in all binstubs[1]:

  /nix/store/z115...-ruby-3.3.4/lib/ruby/3.3.0/bundler/definition.rb:450:in
  `validate_platforms!': Your bundle only supports platforms [] but your
  local platform is arm64-darwin. Add the current platform to the
  lockfile with `bundle lock --add-platform arm64-darwin` and try again.

Note however that the Gemfile.lock file should not be tracked by Git, as
instructed in the documentation for GitHub Pages[2]. This is because the
builder image[3] comes with all the allowed Gem dependencies of
github-pages pre-installed, and doesn't offer facilities for installing
additional Gems.

[1]: https://github.com/inscapist/ruby-nix/blob/v0.2.1/modules/ruby-env/gen-bin-stubs.rb#L52
[2]: https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/creating-a-github-pages-site-with-jekyll
[3]: https://github.com/actions/jekyll-build-pages

README.md

@@ -24,6 +24,10 @@ covered by [ISC][isc].
 
 ### Using Nix
 
+```sh
+nix develop --command bundle-lock
+```
+
 ```sh
 nix develop --command jekyll serve --drafts
 ```