Running `clang-format` locally [#242]

Apparently everyone but me knows how to do that; I'm now documenting it for my
own future use.

.github/workflows/clang-format.yml

@@ -13,6 +13,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+    # If updating these steps, please also correspondingly update '../../CONTRIBUTING.md', "Running `clang-format` locally".
     - uses: actions/checkout@v2
     - name: Copy .clang-format file
       run: cp contrib/clang-format .clang-format

CONTRIBUTING.md

@@ -40,7 +40,15 @@ These will be imported into a GitHub PR to follow the normal review process, and
 
 * Where possible please add test cases to `gcc/testsuite/rust/` for all PRs. Some issues may not be testable via dejagnu/automation such as debug dump changes.
 
-* PR's cannot be merged untill clang format and the build and tests pass.
+* PRs won't be merged until `clang-format` and the build and tests pass.
+
+** Running `clang-format` locally
+
+... corresponding to what the _Clang Format Lint_ (`.github/workflows/clang-format.yml`) is doing, with `clang-format-10` being available locally, and avoiding the Docker overhead.
+
+    $ wget 'https://github.com/DoozyX/clang-format-lint-action/raw/v0.11/run-clang-format.py'
+    $ cp contrib/clang-format .clang-format
+    $ python3 run-clang-format.py --clang-format-executable clang-format-10 --recursive --extensions h,cc gcc/rust/
 
 * Please take the time to create good git commit messages see the existing format of them in the git log or refer to something like: https://chris.beams.io/posts/git-commit/