Update README.md to include documentation for UX (interaction) design...

Update README.md to include link to documentation for UX (interaction) design contributors. (Please check if i formatted the link in the right way to be sure it works) Goal: to make UX research more accessible on the Manyverse Gitlab project. It is now quite hard to find, while information for code contributors is on the front page. It would be nice to be able to attract UX contributors as well.