Add CONTRIBUTE file

This change adds some developper documentation.

Change-Id: I5b366b1e30a6e4c095ce859587261f76b75d4c3e

CONTRIBUTE.md

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+# Developper documentation
+
+The zuul operator is a container application that manages a
+zuul service described by a high level description (named CR).
+Its goal is to create the kubernetes resources and to perform
+the runtime operations.
+
+To describe the kubernetes resources such as Deployment and Service,
+the zuul operator uses the Dhall language to convert the CR input
+into the kubernetes resources.
+
+To apply and manage the resources, the zuul operator uses Ansible
+through the operator-framework to execute the roles/zuul when
+a Zuul CR is requested.
+
+The following sections explain how to evaluate the Dhall configuration
+and the Ansible task locally, outside of a kubernetes pod.
+This simplifies the development and contribution process.
+
+
+## Setup tools
+
+Install the `dhall-to-yaml` and `yaml-to-dhall` tool by following this tutorial:
+https://docs.dhall-lang.org/tutorials/Getting-started_Generate-JSON-or-YAML.html#installation
+
+Or use the zuul-operator image:
+
+```bash
+CR="podman"
+alias dhall-to-yaml="$CR run --rm --entrypoint dhall-to-yaml -i docker.io/zuul/zuul-operator"
+alias yaml-to-dhall="$CR run --rm --entrypoint yaml-to-dhall -i docker.io/zuul/zuul-operator"
+```
+
+## Evaluate the dhall expression manually:
+
+First you need to convert a CR spec to a dhall record, for example using the test file `playbooks/files/cr_spec.yaml`:
+
+```bash
+INPUT=$(yaml-to-dhall "(./conf/zuul/input.dhall).Input.Type" < playbooks/files/cr_spec.yaml)
+```
+
+Then you can evaluate the resources function, for example to get the scheduler service:
+
+```bash
+dhall-to-yaml --omit-empty --explain <<< "(./conf/zuul/resources.dhall ($INPUT)).Components.Zuul.Scheduler"
+```
+
+Or get all the kubernetes resources:
+
+```bash
+dhall-to-yaml --omit-empty <<< "(./conf/zuul/resources.dhall ($INPUT)).List"
+```
+
+## Run the ansible roles locally
+
+Given a working `~/.kube/config` context, you can execute the Ansible roles directly using:
+
+```bash
+ansible-playbook -v playbooks/files/local.yaml
+```
+
+Then cleanup the resources using:
+
+```bash
+ansible-playbook -v playbooks/files/local.yaml -e k8s_state=absent
+```