feat: set message for multiple revsets

Allows to set the revision message of multiple revisions by passing
them as arguments. This only supports simple revisions, such as `@`,
`@-`, `xs`, and so on. Comple revisions such as `@..@-` are not
supported.

Fixes: #5

README.md

@@ -24,6 +24,16 @@ jj-cz
 The tool detects whether you're in a Jujutsu repository, guides you
 through the commit message, and applies it to your current change.
 
+You can also set the revision message of a few revisions at once, or
+target a single revision other than the current one.
+
+```sh
+jj-cz @- xs develop
+```
+
+No explicit revision is simply the equivalent of `jj-cz @`, like
+`jj desc`.
+
 ## Requirements
 
 - A Jujutsu repository
@@ -41,8 +51,44 @@ what `jj-cz` alone would be good for without `jj`.
 | 130  | Interrupted                                          |
 
 ## Installation
+### From crates.io
+Simply run the following command:
 
-You can install jj-cz with Cargo by building it from source.
+```
+cargo install jj-cz
+```
+
+Done! `jj-cz` is now available!
+
+### With Nix Flakes
+Notice how there’s a `flake.nix` file? This means you can run the
+project using this repository as one of your flakes inputs. In fact,
+that’s how I install it in my own NixOS configuration! Add this
+repository to your configuration:
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+
+    jj-cz = {
+      url = "git+https://labs.phundrak.com/phundrak/jj-cz";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+}
+```
+
+And tadah! you can now install
+`inputs.jj-cz.packages.${pkgs.stdenv.hostPlatform.system}.default`
+among your other packages. Take a look at my
+[`jujutsu.nix`](https://labs.phundrak.com/phundrak/nix-config/src/branch/main/users/modules/dev/vcs/jujutsu.nix)
+module if you need some inspiration.
+
+### From source
+
+You can also install `jj-cz` with Cargo by building it from source.
+Just make sure Rust is available on your machine (duh!).
 
 ```sh
 cargo install --path .