Added “Run on YunoHost”

Pixelfed is super simple to self-host even for people with limited technical skills, thanks to YunoHost. Such a great advantage (which very few Fediverse social media have) should be made more visible, as in the [Wallabag repository](https://github.com/wallabag/wallabag#run-on-yunohost '“Run on YunoHost“ on Wallabag README on GitHub')