add clang-format pre-commit hook (XRPLF#4599)

* Add a new YAML file (.pre-commit-config.yaml) to set up pre-commit
  hook for clang-format
  * The pre-commit hook is opt-in and needs to be installed in order to
    run automatically
* Update CONTRIBUTING.md with instructions on how to set up and use the
  clang-format linter

Automating the process of running clang-format before committing code
helps to save time by removing the need to fix formatting issues later.

This is a tooling improvement and doesn't change C++ code.

.pre-commit-config.yaml

@@ -0,0 +1,6 @@
+# .pre-commit-config.yaml
+repos:
+- repo: https://github.com/pre-commit/mirrors-clang-format
+  rev: v10.0.1
+  hooks:
+  - id: clang-format

CONTRIBUTING.md

@@ -121,6 +121,11 @@ this:
 You can format individual files in place by running `clang-format -i <file>...`
 from any directory within this project.
 
+You can install a pre-commit hook to automatically run `clang-format` before every commit:
+```
+pip3 install pre-commit
+pre-commit install
+```
 
 ## Avoid