Add https://search.404.fo

Close #697 
From @cryphixer

Author: ononoki1

searxinstances/instances.yml

@@ -19,6 +19,7 @@ https://priv.au: {}
 https://s.datuan.dev: {}
 https://s.mble.dk: {}
 https://search.080609.xyz: {}
+https://search.404.fo: {}
 https://search.canine.tools: {}
 https://search.catboy.house: {}
 https://search.citw.lgbt: {}