#70 Add a wiki page (and a link to that page in README.md) or something similar to let people know how to make a pull request

I know it may seem simple or obvious, but that's easy to make and it makes it less probable that one makes a mistake or forgets something.

Many people maybe don't know that the games must be alphabetically ordered, may forget to update the CHANGELOG.md file...

I was going to do something of an organizational refactor - I will introduce a CONTRIBUTING.md to address this as part of this change, mentioning precisely this information. Thank you for the suggestion!