Add readme

README.md

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+# Install Rust Toolchain
+
+This GitHub Action installs a Rust toolchain using rustup. It is designed for
+one-line concise usage.
+
+## Example workflow
+
+```yaml
+name: test suite
+on: [push, pull_request]
+
+jobs:
+  test:
+    name: cargo test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo test --all-features
+```
+
+The selection of Rust toolchain is made based on the particular @rev of this
+Action being requested. For example "dtolnay/rust-toolchain@nightly" pulls in
+the nightly Rust toolchain, while "dtolnay/rust-toolchain@1.42.0" pulls in
+1.42.0.
+
+## Inputs
+
+All inputs are optional.
+
+| Name         | Description                                                                                                                        |
+| ------------ | -----------------------------------------------------------------------------------------------------------------------------------|
+| `toolchain`  | Rustup toolchain specifier, ex. `stable`, `nightly`, `1.42.0`. **Important: the default is to match the @rev as described above.** |
+| `target`     | Additional target support to install, ex. `wasm32-unknown-unknown`                                                                 |
+| `components` | Comma-separated string of additional components to install, ex. `clippy, rustfmt`                                                  |
+
+## License
+
+The scripts and documentation in this project are released under the [MIT
+License].
+
+[MIT License]: LICENSE