doc: update contrib docs

Author: github-actions

_contrib/contributing.md

@@ -13,3 +13,13 @@ You want to contribute? You're awesome! Don't know where to start? Check the [li
 
 - [Submitting a Pull Request](pull-requests.md#submitting-a-pull-request)
 - [Commit Guidelines](commits.md)
+
+## Cargo fmt
+`cargo fmt --all` does not work in hyper. Please use the following commands:
+```txt
+# Mac or Linux
+rustfmt --check --edition 2018 $(git ls-files '*.rs')
+
+# Powershell
+Get-ChildItem . -Filter "*.rs" -Recurse | foreach { rustfmt --check --edition 2018 $_.FullName }
+```