Add README

README.md

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+# Install Rust Toolchain
+
+This GitHub Action installs a Rust toolchain using rustup.
+It further integrates into the ecosystem.
+Caching for Rust tools and build artifacts is enabled.
+Environment variables are set to optimize the cache hits.
+[Problem Matchers] are provided for build messages (cargo, clippy) and formatting (rustfmt).
+
+The action is heavily inspired by *dtolnay*'s <https://github.com/dtolnay/rust-toolchain> and extends it with further features.
+
+## Example workflow
+
+```yaml
+name: "Test Suite"
+on:
+  push:
+  pull_request:
+
+jobs:
+  test:
+    name: cargo test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions-rust-lang/setup-rust-toolchain@v1
+      - run: cargo test --all-features
+```
+
+## Inputs
+
+All inputs are optional.
+
+| Name         | Description                                                                       | Default |
+| ------------ | --------------------------------------------------------------------------------- | ------- |
+| `toolchain`  | Rustup toolchain specifier e.g. `stable`, `nightly`, `1.42.0`.                    | stable  |
+| `target`     | Additional target support to install e.g. `wasm32-unknown-unknown`                |         |
+| `components` | Comma-separated string of additional components to install e.g. `clippy, rustfmt` |         |
+
+## Outputs
+
+| Name             | Description                               |
+| ---------------- | ----------------------------------------- |
+| `rustc-version`  | Version as reported by `rustc --version`  |
+| `cargo-version`  | Version as reported by `cargo --version`  |
+| `rustup-version` | Version as reported by `rustup --version` |
+
+## License
+
+The scripts and documentation in this project are released under the [MIT
+License].
+
+[MIT License]: LICENSE
+[Problem Matchers]: https://github.com/actions/toolkit/blob/main/docs/problem-matchers.md