Add link to /r/reproduciblebuilds

Bernhard M. Wiedemann wrote to the r-b general list:

"because there are people out there that might not like Mailing Lists for discussion or getting the latest updates, opened https://www.reddit.com/r/reproduciblebuilds/ "

to which I replied: that's awesome, even though I'm not a reddit user myself, I know many people and esp. developers hang out there. Now I just wonder:

where to make this information available on https://reproducible-builds.org/

Creating this issue to track and fix this :)