[Tagging] horse rental

[Hufkratzer]

As far as horses are concerned, in
https://lists.openstreetmap.org/pipermail/tagging/2018-August/038781.html
I wrote: "If the horse rental is only a secondary activity it may be 
tagged horse_rental=yes (with underline, currently used once) instead of 
horse:rental=yes (with colon), analogously to what is written on the 
wiki pages for amenity=animal_boarding and amenity=animal_breeding, for 
example."