Add a rust-toolchain.toml file (#538)

.github/workflows/ci.yaml

@@ -15,7 +15,6 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: 1.63.0
       - uses: actions/cache@v3
         env:
           cache-name: cache-cargo

README.md

@@ -640,7 +640,7 @@ extend-ignore = [
 
 ## Development
 
-Ruff is written in Rust (1.63.0). You'll need to install the [Rust toolchain](https://www.rust-lang.org/tools/install)
+Ruff is written in Rust (1.64.0). You'll need to install the [Rust toolchain](https://www.rust-lang.org/tools/install)
 for development.
 
 Assuming you have `cargo` installed, you can run:

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "1.64.0"