Document requirement for VS2015.
Document requirement for VS2015.

Fixes #105.
arvidsson/entityx,alecthomas/entityx,jaynus/entityx,jaynus/entityx,jaynus/entityx,alecthomas/entityx,jarrettchisholm/entityx,drewet/entityx,jarrettchisholm/entityx,jarrettchisholm/entityx,arvidsson/entityx,Bablawn3d5/entityx,Bablawn3d5/entityx,Bablawn3d5/entityx,arvidsson/entityx,alecthomas/entityx,drewet/entityx,drewet/entityx