Fixes #423 (#428)

CONTRIBUTING.md

@@ -0,0 +1,32 @@
+# Contributing
+
+`docker-mailserver` is OpenSource. That means that you can contribute on enhancements, bug fixing or improving the documentation in the Wiki.
+
+## Open an issue
+
+When opening an issue, please provide details use case to let the community reproduce your problem.
+Please start the mail server with env `DMS_DEBUG=1` and paste the ouput into the issue.
+
+## Pull Requests
+
+#### Project architecture
+
+    ├── config                    # User: personal configurations
+    ├── target                    # Developer: default server configuration, used when building the image
+    └── test                      # Developer: integration tests to check that everything keeps working
+
+#### Development Workflow
+
+The development workflow is the following:
+
+- Fork project and clone your fork
+- Create a branch using `git checkout -b branch_name` (you can use `issue-xxx` if fixing an existing issue)
+- Code :-)
+- Add integration tests in `test/tests.bats`
+- Use `make` to build image locally and run tests
+- Document your improvements
+- [Commit](https://help.github.com/articles/closing-issues-via-commit-messages/), push and make a pull-request
+- Pull-request is automatically tested on Travis
+- When tests are green, your branch is merged into `master`
+- `master` is automatically tested on Travis
+- Docker builds a new `latest` image