[Tagging] RFC for mounting_block

[Warin]

Hi,

Following the draft for the feature mounting block (traditionally used to mount a horse) I have moved this to a 'request for comments' stage.
I think most have had an input but here it is to meet the rules.

https://wiki.openstreetmap.org/wiki/Proposed_features/mounting_block

Definition: An aid to mounting a horse, usually steps.

Value:mounting_block

Key: At this stage yet to be determined - by voting! This will be the 
first vote. Then there should be another vote for the whole thing. 
Rational: exists in the data base but is inconsistent for the key and 
value used. This seeks to coordinate the tagging of these features.