Add a buildifier pre-commit hook (#481)

* Add a buildifier pre-commit hook

Matches the one just landed in rules_apple: bazelbuild/rules_apple@61bc7c0

* Update .pre-commit-config.yaml

Co-authored-by: Jonathon Belotti <[email protected]>

Co-authored-by: Jonathon Belotti <[email protected]>

Author: alexeagle

.bazelci/presubmit.yml

@@ -1,6 +1,7 @@
 ---
 buildifier:
   version: latest
+  # keep this argument in sync with .pre-commit-config.yaml
   warnings: "all"
 all_targets: &all_targets
     build_targets:

.pre-commit-config.yaml

@@ -0,0 +1,13 @@
+# See CONTRIBUTING.md for instructions.
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+  - repo: https://github.com/keith/pre-commit-buildifier
+    rev: 4.0.1.1
+    hooks:
+      - id: buildifier
+        args: &args
+          # Keep this argument in sync with .bazelci/presubmit.yaml
+          - --warnings=all
+      - id: buildifier-lint
+        args: *args

CONTRIBUTING.md

@@ -3,6 +3,19 @@
 We'd love to accept your patches and contributions to this project. There are
 just a few small guidelines you need to follow.
 
+## Formatting
+
+Starlark files should be formatted by buildifier.
+We suggest using a pre-commit hook to automate this.
+First [install pre-commit](https://pre-commit.com/#installation),
+then run
+
+```shell
+pre-commit install
+```
+
+Otherwise the Buildkite CI will yell at you about formatting/linting violations.
+
 ## Contributor License Agreement
 
 Contributions to this project must be accompanied by a Contributor License