doc/crystal: Update to mention shard.lock file generation

doc/languages-frameworks/crystal.section.md

···
 
 This section uses [Mint](https://github.com/mint-lang/mint) as an example for how to build a Crystal package.
 
-If the Crystal project has any dependencies, the first step is to get a `shards.nix` file encoding those. Get a copy of the project and go to its root directory such that its `shard.lock` file is in the current directory, then run `crystal2nix` in it
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+If the Crystal project has any dependencies, the first step is to get a `shards.nix` file encoding those. Get a copy of the project and go to its root directory such that its `shard.lock` file is in the current directory. Executable projects should usually commit the `shard.lock` file, but sometimes that's not the case, which means you need to generate it yourself. With an existing `shard.lock` file, `crystal2nix` can be run.
 ```bash
 $ git clone https://github.com/mint-lang/mint
 $ cd mint
 $ git checkout 0.5.0
+$ if [ ! -f shard.lock ]; then nix-shell -p shards --run "shards lock"; fi
 $ nix-shell -p crystal2nix --run crystal2nix
 ```