Add pre-commit config.

.pre-commit-config.yaml

+-   repo: git://github.com/pre-commit/pre-commit-hooks
+    sha: cf550fcab3f12015f8676b8278b30e1a5bc10e70
+    hooks:
+    -   id: check-added-large-files
+    -   id: check-json
+    -   id: check-xml
+    -   id: check-yaml
+    -   id: check-merge-conflict