Updated template for PRs

[JohanMabille]

Please check if your PR fulfills these requirements

• The title and the commit message(s) are descriptive
• Small commits made to fix your PR have been squashed to avoid history pollution
• Tests have been added for new features or bug fixes
• API of new functions and classes are documented
• If you PR introduces backward incompatible changes, update the version number
  in include/xtl/xtl_config.hpp according to the following rules:
  • if XTL_VERSION_PATCH is already 0-dev, you have nothing to do
  • otherwise, set XTL_VERSION_PATCH to 0-dev and increase XTL_VERSION_MINOR by 1