Add readme instructions

Author: M-arcus

Dockerfile

@@ -60,4 +60,7 @@ WORKDIR /app
 CMD echo 'PHP Code Style Swissknife Docker' && \
     echo '--------------------------------' && \
     echo 'Try the following commands:' && \
-    ls /composer/vendor/bin
+    echo 'docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest ecs' && \
+    echo 'docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest phpstan' && \
+    echo 'docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest phpmd' && \
+    echo 'docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest rector'

README.md

@@ -1 +1,20 @@
 # PHP-Codestyle-Swissknife-Docker
+
+This is a docker image that contains the following tools:
+- [PHPStan](https://phpstan.org/)
+- [PHP-CS-Fixer](https://cs.symfony.com/) / [EasyCodingStandard](https://github.com/easy-coding-standard/easy-coding-standard)
+- [PHP Mess Detector](https://phpmd.org/)
+- [Rector](https://getrector.com/)
+
+## Usage
+
+```bash
+docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest ecs ...
+docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest phpstan ...
+docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest phpmd ...
+docker run --rm -v $(pwd):$(pwd) -w $(pwd) -u $(id -u):$(id -g) marcusmu/php-codestyle-swissknife-docker:latest rector ...
+```
+
+Adapted from
+ - https://github.com/aragon999/phpstan-shopware-docker/
+ - https://github.com/shyim/php-cs-fixer-docker