Create Github Action for easy translations of files in pipelines

[mondeja]

Use md2po2md and submit changes in pull requests

• Add a real usage example with the README of this project in GitHub workflows installing mdpo, running it from the command line with --check option and submitting the changes to the current pull request.

• Run on pull requests.
• Specify parameters with inputs.
• Exit with non zero code if files have been changed using the --check option by default.
• Use --no-empty-msgstr and --no-obsolete to avoid linting tools (see Add options to avoid linting tools for translations #252)