Add clang-format-check GitHub action

.github/workflows/clang-format-check.yml

@@ -0,0 +1,12 @@
+name: Clang Format Checker
+on: [push]
+jobs:
+  clang-format-checking:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: RafikFarhad/clang-format-github-action@v2.1.0
+        with:
+          style: file
+          sources: "apps/**/*.h,apps/**/*.c,examples/*.c,includes/**/*.h,src/**/*.h,src/**/*.c,tests/**/*.h,tests/**/*.c"
+          excludes: "apps/shared/iccjpeg.*"

README.md

@@ -66,7 +66,9 @@ Use [clang-format](https://clang.llvm.org/docs/ClangFormat.html) to format the C
 sources from the top-level folder:
 
 ```sh
-clang-format -i apps/*.c examples/*.c include/avif/*.h src/*.c tests/*.c
+clang-format -i apps/*.c apps/shared/avifjpeg.* apps/shared/avifpng.* \
+                apps/shared/avifutil.* apps/shared/y4m.* \
+                examples/*.c include/avif/*.h src/*.c tests/*.h tests/*.c
 ```
 
 Use [cmake-format](https://github.com/cheshirekow/cmake_format) to format the