Update contributing guidelines (#14)

Author: cytopia

CONTRIBUTING.md

@@ -20,29 +20,43 @@ The above command will guide you through a couple of questions and will auto-gen
 
 * your project directory with Dockerfiles
 * an entry in the main README.md
-* GitHub action pipelines.
+* GitHub action pipelines
+* GitHub code ownership
 
 Once this is done, you can start refining your Dockerfiles, update your project README.md, create according tests if needed and PR it for initial review.
 
 
+## How do I build my own flavour locally?
+
+If you want to test your build locally do so as follows:
+```
+# Defaults to linux/amd64
+make build FLAVOUR=<flavour-name> VERSION=<php-version>
+
+# Explicitly build linux/arm64
+make build FLAVOUR=<flavour-name> VERSION=<php-version> ARCH=linux/arm64
+```
+
+
 ## Becoming a maintainer
 
-As a maintainer, you will need create a flavour for PHP 5.3 - PHP 8.2 for the `amd64` and `arm64` architecture.
+As a maintainer, you must create a working flavour for all PHP versions (PHP 5.3 to PHP 8.2) and for both architectures (`amd64` and `arm64`).
 
 ### Benefits
 
 If you plan on adding your own flavoured images to this repository, you will gain the following benefits:
 
 * namespaced flavour
 * your customized images
-* automatic nightly builds for always up-to-date images
+* automatic nightly builds for always up-to-date images pullable from [Dockerhub](https://hub.docker.com/r/devilbox/php-fpm-community)
+* useable with the [Devilbox](https://github.com/cytopia/devilbox)
 
 ### Responsibilities
 
-By doing so, you will also have to take the continuous responsible to:
+By doing so, you will also have to take the continuous responsibility to:
 
 * be the GitHub code owner of your flavour
-* ensure your Docker image builds pass
+* ensure the builds of your Docker images pass
 * work on reported issues on your flavour
 * ensure tests exist for anything specific you add to your flavour
 * continuously groom your project and review assigned pull requests
@@ -58,7 +72,8 @@ It will basically become your project.
     # For the following to work, you must have built all images locally.
 
     FLAVOUR=devilbox
+    ARCH=linux/amd64
     for i in 5.3 5.4 5.5 5.6 7.0 7.1 7.2 7.3 7.4 8.0 8.1 8.2; do \
-      ./tests/bin/gen-readme.sh devilbox/php-fpm-community linux/amd64 ${i}-${FLAVOUR} ${FLAVOUR} ${i}; \
+      ./tests/bin/gen-readme.sh devilbox/php-fpm-community ${ARCH} ${i}-${FLAVOUR} ${FLAVOUR} ${i}; \
     done
     ```