By committing or contributing data/files to the Tuxemon project, or any sub- projects (the "Projects"), you agree to license your code under the GNU General Public License version 3 and any later version (the "License"). In particular, you guarantee that you have acquired all necessary legal rights from possible other copyright holders to license the contributions.
In any case, any contributions to the Projects must be able to be re-distributed by the maintainers or others as part of the Git repository and cloned repositories, in compiled binaries, and any other ways permitted by the License.
Any of the issues on the Tuxemon issue list are things we are looking to have implemented in Tuxemon. Feel free to fork the project and taking a shot at anything on the list! If you have a suggestion for a feature that's not on the current issues list, post a topic on the Programming section of the Tuxemon Forums and discuss ways we might implement it in the game.
Tuxemon uses the "Fork & Pull" method for code contributions. The fork & pull model lets anyone fork an existing repository and push changes to their personal fork without requiring access be granted to the source repository. The changes must then be pulled into the source repository by the project maintainer. This model reduces the amount of friction for new contributors and is popular with open source projects because it allows people to work independently without upfront coordination.
When you're ready to make a pull request, submit your pull to the "development" branch. The "master" branch is used for releases.