Update readme

Author: jonasbb

README.md

@@ -1,2 +1,37 @@
-# rustfmt
-Check formatting of Rust code with rustfmt
+# Run `rustfmt`
+
+Run `cargo fmt --all` and report all formatting differences in a nice overview.
+It works best in combination with [`actions-rust-lang/setup-rust-toolchain`] for [problem matcher] highlighting.
+
+Execution Summary:
+
+![The action reports any formatting issues found by rustfmt.](./imgs/rustfmt-results.png)
+
+Problem Matcher:
+
+![Annotation highlighting formatting differences.](./imgs/rustfmt-problem-matcher.png)
+
+## Example workflow
+
+```yaml
+name: "Test Suite"
+on:
+  push:
+  pull_request:
+
+jobs:
+  formatting:
+    name: cargo fmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      # Ensure rustfmt is installed and setup problem matcher
+      - uses: actions-rust-lang/setup-rust-toolchain@v1
+        with:
+          components: rustfmt
+      - name: Rustfmt Check
+        uses: actions-rust-lang/rustfmt@v1
+```
+
+[`actions-rust-lang/setup-rust-toolchain`]: https://github.com/actions-rust-lang/setup-rust-toolchain
+[problem matcher]: https://github.com/actions/toolkit/blob/main/docs/problem-matchers.md