A few tasks related to the repository itself, not to the code.

* Ruby 2.3 support has been removed. The servers now run Ruby 2.4 and we don't need the testing matrix anymore. The application may still work on Ruby 2.3, but it is not tested anymore and there are no confidence on whether it will work or not.

* Introduces CONTRIBUTING.md file for the contributing guidelines for this project, such as how to send an issue or a pull request. Updates the README.md file to link to the contributing file.

* Introduces ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md so that GitHub shows some text to guide GitHub users as they are writing an issue or a pull request. There is more information in the CONTRIBUTING.md file, but the templates have the most important things I want them to know.