Install shell completion for nix (#8)

When installing via nix, the shell completions are generated as a post install step.

docs/index.md

@@ -20,6 +20,32 @@ form of linux, try `*-amd64` or `*-aarch64`. Last for Windows use the
 Once downloaded, you can simply execute it without any further
 installation step.
 
+#### shell completion
+
+For convenience, the cli tool can generate completion commands for
+several shells. You can use it for inclusion in your `.bashrc` or
+similar setups.
+
+For example:
+
+``` bash rnk:silent
+rnk shell-completion --shell bash
+```
+
+will generate the completions for bash. These have to be "sourced"
+into into your current shell:
+
+``` bash
+eval "$(rnk shell-completion --shell bash)"
+```
+
+Add this line to your `.bashrc` to have these completions available
+when you enter bash.
+
+With this enabled, when you type `rnk <tab>` you will be presented
+with possible options, that are narrowed down the more letters you
+type.
+
 ### Nix User
 
 If you are a nix user and have flakes enabled, you can install rnk
@@ -34,6 +60,9 @@ If you want to try it out without installing:
 nix run github:SwissDatascienceCenter/renku-cli
 ```
 
+When installing the package via `install` or including it into your
+NixOS configuration, the shell completions are already installed.
+
 ### Debian/Ubuntu User
 
 TODO

flake.nix

@@ -55,6 +55,7 @@
           [
             # Add additional build inputs here
             pkgs.openssl
+            pkgs.installShellFiles
           ]
           ++ lib.optionals pkgs.stdenv.isDarwin [
             # Additional darwin specific inputs can be set here
@@ -85,6 +86,12 @@
           inherit cargoArtifacts;
           # additional arguments to cargo for building the app, --release is already there
           cargoExtraArgs = "";
+          postInstall = ''
+            for shell in fish zsh bash; do
+               $out/bin/rnk shell-completion --shell $shell > rnk.$shell
+               installShellCompletion --$shell rnk.$shell
+            done
+          '';
         });
     in {
       checks = {