Update contributing guide

Add recommendations to improve compile times by copying the
`config_fast_builds.toml` file. There was an implicit comment in
the file that isn't easy to find as a first contributor.

Also added a link to the official documentation to compile with
performance optimizations.

CONTRIBUTING.md

@@ -419,6 +419,8 @@ Bevy is actively open to code contributions from community members.
 If you're new to Bevy, here's the workflow we use:
 
 1. Fork the `bevyengine/bevy` repository on GitHub. You'll need to create a GitHub account if you don't have one already.
+    1. Copy `.cargo/config_fast_builds.toml` to `.cargo/config.toml`. Then update the file and follow the general
+  recommendations to [compile with performance optimizations](https://bevyengine.org/learn/quick-start/getting-started/setup/#compile-with-performance-optimizations).
 2. Make your changes in a local clone of your fork, typically in its own new branch.
    1. Try to split your work into separate commits, each with a distinct purpose. Be particularly mindful of this when responding to reviews so it's easy to see what's changed.
    2. Tip: [You can set up a global `.gitignore` file](https://docs.github.com/en/get-started/getting-started-with-git/ignoring-files#configuring-ignored-files-for-all-repositories-on-your-computer) to exclude your operating system/text editor's special/temporary files. (e.g. `.DS_Store`, `thumbs.db`, `*~`, `*.swp` or `*.swo`) This allows us to keep the `.gitignore` file in the repo uncluttered.