Add a contribution guide to the repository

Signed-off-by: Guillaume Lours <[email protected]>

CONTRIBUTING.md

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+# Contributing
+
+Contributions should be made via pull requests. Pull requests will be reviewed
+by one or more maintainers and merged when acceptable.
+
+The goal of the Compose Reference Implementation is to be a complete and simple implementation of [the Compose Specification](https://github.com/compose-spec/compose-spec/).
+A developer should be able by reading the code: 
+ - to understand the main concept of the specification 
+ - have concrete examples to develop his own implementation.
+
+## Successful Changes
+
+We ask that before contributing, please make the effort to ensure you have read the [Compose Specification](https://github.com/compose-spec/compose-spec/blob/master/spec.md) and 
+[Compose Vision](https://github.com/compose-spec/compose-spec/blob/master/VISION.md) to ensure that your change is in keeping with the objectives of Compose. 
+You can coordinate with the maintainers of the project before submitting large proposals 
+or high impact PRs, this will prevent you from doing extra work that may or may not be merged.
+PRs that are just submitted without any prior communication will likely be summarily closed.
+
+While pull requests are the methodology for submitting changes, changes are much more 
+likely to be accepted if they are accompanied by a full justification of what developer 
+problem you are solving. Often times, it helps to first state the problem before presenting 
+solutions.
+
+Typically, the best methods of accomplishing this are to submit an issue, stating the 
+problem. This issue can include a problem statement and a checklist with requirements. 
+If solutions are proposed, alternatives should be listed and eliminated. Even if the 
+criteria for elimination of a solution is frivolous, say so.
+Larger changes typically work best with design documents, these are items which may 
+change the scope or vision for Compose. These should be accompanied with a more detailed 
+overview of the proposal, providing context to the justfication at the time the feature 
+was conceived and can inform future documentation contributions.
+
+## Commit Messages
+
+There are times for one line commit messages and this is not one of them.
+Commit messages should follow best practices, including explaining the context
+of the problem and how it was solved, including in caveats or follow up changes
+required. They should tell the story of the change and provide readers
+understanding of what led to it.
+
+If you're lost about what this even means, please see [How to Write a Git
+Commit Message](http://chris.beams.io/posts/git-commit/) for a start.
+
+In practice, the best approach to maintaining a nice commit message is to
+leverage a `git add -p` and `git commit --amend` to formulate a solid
+changeset. This allows one to piece together a change, as information becomes
+available.
+
+If you squash a series of commits, don't just submit that. Re-write the commit
+message, as if the series of commits was a single stroke of brilliance.
+
+That said, there is no requirement to have a single commit for a PR, as long as
+each commit tells the story. For example, if there is a feature that requires a
+package, it might make sense to have the package in a separate commit then have
+a subsequent commit that uses it.
+
+Remember, you're telling part of the story with the commit message. Don't make
+your chapter weird.
+
+## Sign your work
+
+The sign-off is a simple line at the end of the explanation for the patch. Your
+signature certifies that you wrote the patch or otherwise have the right to pass
+it on as an open-source patch. The rules are pretty simple: if you can certify
+the below (from [developercertificate.org](http://developercertificate.org/)):
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+Then you just add a line to every git commit message:
+
+    Signed-off-by: Joe Smith <[email protected]>
+
+Use your real name (sorry, no pseudonyms or anonymous contributions.)
+
+If you set your `user.name` and `user.email` git configs, you can sign your
+commit automatically with `git commit -s`.
+