Add contribution notes

Author: Greg Bowler

CONTRIBUTING.md

@@ -0,0 +1,28 @@
+How to contribute to php-actions/php-build
+==========================================
+
+To build this image for local development:
+
+```bash
+docker build .
+```
+
+The last line from `docker build` will show the image ID, which identifies the final layer of the image. You can use this ID to interact further.
+
+To launch into the image, within a bash shell:
+
+```bash
+docker run -it $image_id /bin/bash
+```
+
+After making changes, run `docker build .` again to build the extra layers that you've added, making a note of the new image ID.
+
+```bash
+docker tag $new_image_id ghcr.io/php-actions/php-build:$version_number
+```
+
+To deploy the version to Github:
+
+```bash
+docker push ghcr.io/php-actions/php-build:$version_number
+```