Add rust-toolchain.toml to project (#249)

Added a rust-toolchain.toml to the project that matches the current
build system's rust version.

Also includes a section in our CONTRIBUTING.md discussing the reason for
the Rust versioning and how to ensure the correct version is installed.

Work on #241

CONTRIBUTING.md

@@ -49,6 +49,14 @@ git submodule update --init --recursive
 You will likely want to replace `https://github.com/googleforgames/quilkin.git` with your own fork of the repository
 for your development.
 
+### Rust version
+
+We target a specific version of Rust for each build, so we can be deliberate when changing versions to 
+track if there are any performance changes between Rust releases.
+
+Run `rustup show` in the root of the project directory to install the rust toolchain that is standard for Quilkin 
+development.
+
 ### Building
 
 Debug release:

rust-toolchain.toml

@@ -0,0 +1,17 @@
+# Copyright 2021 Google LLC All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+[toolchain]
+channel = "1.47.0"
+components = ["rustfmt", "clippy"]