Initial instructions for contributing

CONTRIBUTING.md

 # Instructions for contributing
 
+This guide contains instructions for collaborators to contribute with
+the `legTools` package.
+
+The general guidelines for contributing to any of the projects in this
+GitLab can be found in [Boas práticas do uso do Git e GitLab no LEG][]
+(in portuguese only).
+
+## Workflow
+
+In `legTools` we use the Gitflow workflow, which can be viewed in this
+two links: [A successful Git branching model][], and
+[Atlassian tutorial][].
+
+Basically, all the ongoing development is made in the `devel`
+branch. New features and bug fixes are made in additional (parallell)
+branches such like `feature/<num>`, where `<num>` is an incremental
+number. Feature branches are merged with the `devel` branch. When a new
+version is supposed to be ready for release, it is moved to the
+`release` branch, where final tests are made to guarantee it is
+stable. Only then it is moved to the `master` branch, where it receives
+a tag with the version number.
+
+This way, users may opt to install the stable version, from the `master`
+branch, or they may install the development version from the `devel`
+branch.
+
+To contribute to the project, please review the commits in the `devel`
+branch to make sure that what you are trying to do is not already being
+done. Then create a new branch, based on the `devel` branch, and make
+changes in this branch. When you are ready to submit your changes,
+please open a Merge Request (MR), and explain in details your
+implementation, and how this could be beneficil to the project.
+
+Also, if you're not sure about how to do something, but have an idea for
+the project, feel free to open an issue and explain what you want from
+there.
+
+
+<!-- links -->
+
+[A successful Git branching model]: http://nvie.com/posts/a-successful-git-branching-model/
+[Atlassian tutorial]: https://www.atlassian.com/git/tutorials/comparing-workflows/feature-branch-workflow
+[Boas práticas do uso do Git e GitLab no LEG]: http://git.leg.ufpr.br/leg/gitlab-rautu/blob/master/CONTRIBUTING.md