[Tagging] Proposed tag value: surface=bare_rock

[Mateusz Konieczny]

Currently there is no good surface value to tag paths with bare rock as
surface. Such surface occurs typically on paths through mountains.

Among more common tags used for this purpose are
surface=stone/rocky/rock. These are not clear and may mean also
different things. Some people also used surface=limerock/granite/...
but it would be better to use one tag for any kind of material and
specify type of rock by additional tags (material key?) - it would make
easier to process these tags. Asking data consumers to process growing
list of rock types is a poor idea.

Examples of places that may be tagged this way:
http://commons.wikimedia.org/wiki/Category:Kriv%C3%A1%C5%88#/media/File:Na_vrchole_Kriv%C3%A1%C5%88a.jpg
https://commons.wikimedia.org/wiki/File:Cinibulkova_stezka,_mezi_Bludi%C5%A1t%C4%9Bm_a_Rovinou_%2810%29,_tesan%C3%A9_schody_s_%C5%99et%C4%9Bzy.jpg
http://commons.wikimedia.org/wiki/Category:Kriv%C3%A1%C5%88#/media/File:Krywan.JPG
http://wiki.openstreetmap.org/wiki/File:Absamer_klettersteig.jpg

See also
http://wiki.openstreetmap.org/wiki/Talk:Key:surface#surface.3Dstones.2C_surface.3Drock
that proposed using surface=rock for this purpose.