New, easier joining method

[3x]

To make joining @eu easier for everyone, I'm changing the method used to join the organization. If you would like to join, please open a new issue and add the "join" label!

The purpose of the list.txt file was to keep a log of who has joined, but issues would do the same in an arguably better manner, since username changes would be natively reflected by GitHub. Also, I won't need to keep patching and merging PRs. :)