ToDo: Guide of BFO extension wikipage

[zhengj2007]

Currently BFO hierarchy is deliberately as small as possible to allow (what we now believe to be) universal application. That is to say, for any given entity there is a BFO category (below ‘entity’) under which it (non-trivially) falls. However there are for several categories in BFO there are sub-categories which are formal (= domain-neutral) and which are useful for certain purposes. A subcategory of quality, for instance, might be quality-aggregate (an instance of which would be the quality pattern of this chessboard). The strategy for dealing with such extension categories is analogous to the strategy encapsulated in the RO relation repository. That is to say, we will create a wikipage where extension categories can be accessed, including definitions and examples of use.