Add CONTRIBUTING.md file (#114)

* Contributing file

* Update CONTRIBUTING.md

Co-authored-by: Orfeas Kourkakis <orfeas.kourkakis@canonical.com>

---------

Co-authored-by: Orfeas Kourkakis <orfeas.kourkakis@canonical.com>

CONTRIBUTING.md

@@ -0,0 +1,64 @@
+# Contributing
+
+## Overview
+
+This document outlines the processes and practices recommended for contributing enhancements to ``admission-webhook``.
+
+## Talk to us First
+
+Before developing enhancements to this charm, you should [open an issue](/../../issues) explaining your use case. If you would like to chat with us about your use-cases or proposed implementation, you can reach us at [MLOps Mattermost public channel](https://chat.charmhub.io/charmhub/channels/mlops-documentation) or on [Discourse](https://discourse.charmhub.io/).
+
+## Pull Requests
+
+Please help us out in ensuring easy to review branches by rebasing your pull request branch onto the `main` branch. This also avoids merge commits and creates a linear Git commit history.
+
+All pull requests require review before being merged. Code review typically examines:
+  - code quality
+  - test coverage
+  - user experience for Juju administrators of this charm.
+
+## Recommended Knowledge
+
+Familiarising yourself with the [Charmed Operator Framework](https://juju.is/docs/sdk) library will help you a lot when working on new features or bug fixes.
+
+## Build Charm
+
+To build ``admission-webhook`` run:
+
+```shell
+charmcraft pack
+```
+
+## Developing
+
+You can use the environments created by `tox` for development. For example, to load the `unit` environment into your shell, run:
+
+```shell
+tox --notest -e unit
+source .tox/unit/bin/activate
+```
+
+### Testing
+
+Use tox for testing. For example to test the `integration` environment, run:
+
+```shell
+tox -e integration
+```
+
+See `tox.ini` for all available environments.
+
+### Deploy
+
+```bash
+# Create a model
+juju add-model dev
+# Enable DEBUG logging
+juju model-config logging-config="<root>=INFO;unit=DEBUG"
+# Deploy the charm
+juju deploy ./admission-webhook_ubuntu-20.04-amd64.charm \
+    --resource oci-image=$(yq '.resources."oci-image"."upstream-source"' metadata.yaml)
+
+## Canonical Contributor Agreement
+
+Canonical welcomes contributions to this charm. Please check out our [contributor agreement](https://ubuntu.com/legal/contributors) if you're interested in contributing.