Update install information

- using brew on mac OXS
- avoid redundant info between README and MANUAL
> $ g++-mp-4.8 -print-file-name=libgomp.so
> libgomp.so
> It won't give a path here...

Here: 

$ g++-4.7 -print-file-name=libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/4.7/libgomp.so

$ g++ -print-file-name=libgomp.so
/usr/local/lib/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../lib64/libgomp.so

any workaround?