Initial version

README.md

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+# `rustup toolchain` Action
+
+This GitHub Action installs [Rust toolchain](https://github.com/rust-lang/rustup.rs#toolchain-specification).
+
+Optionally it can set installed toolchain as a default and as an override for current directory.
+
+## Example workflow
+
+```yaml
+on: [push]
+
+name: build
+
+jobs:
+  check:
+    name: Rust project
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - name: Install nightly
+        uses: actions-rs/toolchain@1
+        with:
+            toolchain: nightly
+            override: true
+```
+
+## Inputs
+
+* `toolchain`: Toolchain name, see [rustup page](https://github.com/rust-lang/rustup.rs#toolchain-specification) for details.\
+  Examples: `stable`, `nightly`, `nightly-2019-04-20`
+* `default`: Set installed toolchain as default (executes `rustup toolchain default {TOOLCHAIN}`)
+* `override`: Set installed toolchain as an override for current directory
+
+Note: `toolchain` input is required.