docs(README): add example image and text

This should help illustrate its use.

README.md

@@ -23,3 +23,37 @@ Requires `nix`, `jq`, `curl`, `sed`, `gunzip`, `tar` and `coreutils` to be in th
 ```
 
 For more details see the [action.yaml](./action.yml) file.
+
+**How it looks:**
+
+[!example comment generated by this action](./example-comment.png)
+
+<details><summary>Example output as text:</summary>
+
+# Flake output sizes
+
+**Definitions:**
+
+- `Name`: the name of the package/configuration.
+- `Size`: the closure size (size on disk/NAR size + all transitive dependencies).
+- `NAR Size`: the size of the build output (package without the dependencies).
+- `[NAR] Size Change`: the amount changed compared to the main branch.
+
+**Tips on reading this data:**
+
+- For NixOS configurations you generally care only about the `Size` (closure size/size on disk).
+  - Reduce the `Size` by disabling unneeded services/default packages.
+- For Packages you care about both the `Size` and the `NAR Size`.
+  - Reduce the `NAR Size` by reducing the size of the build outputs, e.g. don't copy unnecessary data to the $out dir, optimize binaries for size, etc.
+  - Reduce the `Size` by reducing the dependencies (e.g. `buildInputs`).
+  - Don't worry too much about size, some dependencies are deduplicated, e.g. `glibc` adds ~40MiB to the `Size`, but is generally shared by ~every binary on the system, so, chances are, you are already including it from somewhere else and statically linking with e.g. `musl` is not gonna improve things.
+# NixOS Configurations
+
+| Name | Size | Size Change | NAR Size | NAR Size Change |
+|------|-----:|------------:|---------:|----------------:|
+| `gemini` |        11Gi |   -2.4Mi |  28Ki | 0 |
+| `leo` |      1.6Gi | 0 |  25Ki | 0 |
+| `libra` |       9.4Gi |   -2.4Mi |  28Ki | 0 |
+| `taurus` |      7.6Gi | 0 |  34Ki | 0 |
+
+</details>