Update README, only run builds when dockerfiles touched

.github/workflows/ghcr-push.yml

@@ -8,6 +8,8 @@ concurrency:
 on:
   push:
     branches: [ "master" ]
+    paths:
+      - '**/*.dockerfile'
 
 jobs:
   build-16:

README.md

@@ -1,4 +1,15 @@
-# 
+# clang-format
+
+## Quick Run
+
+```bash
+cd your/source/tree
+podman run --rm -v "${PWD}"/src ghcr.io/cwpearson/clang-format-16:latest clang-format ...
+```
+
+Inside the container, the working directory is /src, we use the volume mount to map the host working directory `${PWD}` into `/src`: `-v "${PWD}"/src`.
+
+## Building the Image Locally
 
 ```bash
 podman build -f clang-format-16.dockerfile -t clang-format-16:latest