Extend usage manual
Extend usage manual

Describes to ways of using metaSMT
* writing source code in the toolbox folder
* referencing metaSMT from an external project using cmake

Fixes #16

hoangmle/metaSMT,agra-uni-bremen/metaSMT,nbruns1/metaSMT,nbruns1/metaSMT,hoangmle/metaSMT,agra-uni-bremen/metaSMT,hoangmle/metaSMT,nbruns1/metaSMT,agra-uni-bremen/metaSMT,finnhaedicke/metaSMT,finnhaedicke/metaSMT,finnhaedicke/metaSMT