Add contributing info

Author: hxtree

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing to phpmarkup
+
+First off, thanks for taking the time to contribute!
+
+For local package development use [Docker](https://www.docker.com/products/docker-desktop):
+
+Build Test container
+```
+git clone https://github.com/Ouxsoft/phpmarkup.git
+cd phpmarkup
+docker build --target test --tag phpmarkup:latest -f Dockerfile .
+docker run -it --mount type=bind,source="$(pwd)"/,target=/application/ phpmarkup:latest composer install
+```
+
+Run Automated Unit Tests using local volume
+```
+docker run -it --mount type=bind,source="$(pwd)"/,target=/application/ phpmarkup:latest composer test
+```
+
+Run Automated Benchmark Tests using local volume
+```
+docker run -it --mount type=bind,source="$(pwd)"/,target=/application/ phpmarkup:latest ./vendor/bin/phpbench run tests/src/Benchmark --report=default
+```
+
+Rebuild Docs
+```
+docker build --target docs --tag phpmarkup:docs-latest -f Dockerfile .
+docker run -it --mount type=bind,source="$(pwd)"/docs,target=/app/docs phpmarkup:docs-latest bash -c "doxygen Doxyfile && doxyphp2sphinx Ouxsoft::phpmarkup"
+```