Add Contributing Info (#1309)

* Add a CONTRIBUTING.md

* Point to book. Add tools/ci script.

* Update contributing link to point to book

.github/CONTRIBUTING.md

@@ -0,0 +1,4 @@
+# Contributing
+
+Please see the [Contributing Code](https://bevyengine.org/learn/book/contributing/code/) section of
+[The Book](https://bevyengine.org/learn/book/introduction/).

README.md

@@ -36,7 +36,8 @@ Bevy is still in the _very_ early stages of development. APIs can and will chang
 
 ## Community
 
-Before contributing or participating in discussions with the community, you should familiarize yourself with our **[Code of Conduct](https://github.com/bevyengine/bevy/blob/master/CODE_OF_CONDUCT.md)**
+Before contributing or participating in discussions with the community, you should familiarize yourself with our **[Code of Conduct](https://github.com/bevyengine/bevy/blob/master/CODE_OF_CONDUCT.md)** and 
+**[How to Contribute](https://bevyengine.org/learn/book/contributing/code/)**
 
 * **[Discord](https://discord.gg/gMUk5Ph):** Bevy's official discord server.
 * **[Reddit](https://reddit.com/r/bevy):** Bevy's official subreddit.

tools/ci

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+
+# This script is intended to mimic some CI behavior that we encourage contributors to run locally.
+# For the actual CI run on GitHub, see the files in .github/workflows/
+
+# Exit when any command fails
+set -e
+
+# Keep track of the last executed command
+trap 'last_command=$current_command; current_command=$BASH_COMMAND' DEBUG
+# Echo an error message before exiting, so you can see exactly what command and what error code
+trap 'echo "\"${last_command}\" command filed with exit code $?."' EXIT
+
+## ACTUAL COMMANDS
+
+# Auto-format Rust files
+cargo +nightly fmt --all
+
+# Run a more intensive linter
+cargo clippy --all-targets --all-features -- -D warnings -A clippy::type_complexity -A clippy::manual-strip