Add new library category

[steve.hickman@…]

At least 2 libraries (Boost.Unit and Boost.ConceptCheck) support higher level software semantics. Used appropriately, they help programmers reason about the meaning behind what code does. The advantage is that they enable compiler driven requirements checking - if the code fails to meet a requirement, it simply doesn't compile correctly! This is a powerful concept. I don't know if most Boost users (or programmers in general) understand how valuable this is. Adding a category on this page (http://www.boost.org/doc/libs/1_60_0/?view=categorized ) might help get that message across.