Add pre-commit to run spotlessApply automatically before git commit

.pre-commit-config.yaml

@@ -0,0 +1,14 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+
+default_install_hook_types:
+   [pre-push]
+
+repos:
+-   repo: local
+    hooks:
+    -   id: spotless-apply
+        name: spotless-apply
+        entry: ./run_spotless_apply.sh
+        language: script
+        stages: [pre-push]

README.md

@@ -87,6 +87,13 @@ Integration tests can be written to call Abacus from non-Kotlin code (i.e., Swif
 
 > ./bump_version.sh
 
+# Auto-lint
+
+Enable pre-commit to auto-lint/auto-format your changes before git commit:
+
+> brew install pre-commit
+> pre-commit install
+
 # How to use
 
 ```

run_spotless_apply.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+./gradlew spotlessApply
+
+exit 0