Add a contributing page

CONTRIBUTING.md

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+**Do you want to contribute ? Please do!**
+
+* [How to help?](#how-to-help?)
+    * [Report issues, propose features](#report-issues-propose-features)
+    * [Solve problems](#solve-problems)
+    * [Say thanks](#say-thanks)
+* [Project workflow](#project-workflow)
+    * [Environment](#environment)
+    * [Development](#development)
+    * [CI/CD](#ci/cd)
+
+# How to help?
+They are several ways to help and improve this project.
+
+## Report issues, propose features
+You think you found a bug ? You think something is missing out ? Check that it has not been [already reported](https://gitlab.com/flwgns-docker/seafile-client/-/issues). If it has not, [report it](https://gitlab.com/flwgns-docker/seafile-client/-/issues/new?issue).
+
+## Solve problems
+You can't wait for a problem to be solved ? Pick one from the [already reported problems](https://gitlab.com/flwgns-docker/seafile-client/-/issues) and solve it. It will be greatly appreciated!
+
+## Say thanks
+One great way to help is simply to say thanks. You can do that by [adding a star to the Docker Hub repository](https://hub.docker.com/r/flowgunso/seafile-client) or [just saying thanks in the project topic on Seafile's forum](https://forum.seafile.com/t/docker-client-to-sync-files-with-containers/8573).
+
+# Project workflow
+The ins and outs of the project, from setting up a local environment for developments to notifying users of changes.
+
+## Environment
+The following is required to develop:
+* Docker engine
+* Bash
+
+## Development
+
+A few utilities Bash scripts are available to operate the project:
+* [build.sh](.utilities/build.sh): build the Docker image and save as a tarball
+* [check.sh](.utilities/check.sh): validate CI pipelines
+* [package_update_notifier.sh](.utilities/package_update_notifier.sh): notify, through a GitLab issue, if a new `seaf-cli` version is available
+* [publish.sh](.utilities/publish.sh): tag and push the image to Docker Hub
+* [test.sh](.utilities/test.sh): run the test suite
+* [update-docker-hub-full-description.sh](.utilities/update-docker-hub-full-description.sh): push the README.md to the Docker Hub
+* [utilities.sh](.utilities/utilities.sh): functions and assets
+
+Following a trunk-based development strategy, all changes must come from a short lived branch and fast-forward merged into master.  
+All commits must be squashed into a single commit referencing a specific issue.
+
+## CI/CD
+Tests are run only on merge requests, but they can be run on demand in a local environment.  
+The [CHANGELOG.md](CHANGELOG.md) and [AUTHORS.md](AUTHORS.md) will be updated on every merge.  
+Only merge resquests associated with a milestone will end up in a new release. The [README.md](README.md) will be updated then. Description pages for Docker Hub and Seafile's forum will be templated then as well and made available as artifacts for a manual update since #10 and #17 are still opened issues.