Update install information

- using brew on mac OXS
- avoid redundant info between README and MANUAL
On Mon, Aug 25, 2014 at 11:36:24AM -0700, Mehdi Amini wrote:
> In pythran/pythran-darwin.cfg:
> > @@ -0,0 +1,14 @@
> > +[sys]
> > +
> > +cppflags = -DENABLE_PYTHON_MODULE -DUSE_GMP -
> DBOOST_SIMD_CONFIG_SUPPORT_POSIX_MEMALIGN
> > +cxxflags = -std=c++11
> > +ldflags =   -lboost_python-mt
> > +            -lgmp
> > +            -lgmpxx
> > +
> > +[user]
> > +
> > +cxx = g++
> Yes but on OSX g++ actually runs clang ;)
> In my pythranrc I overload cxx with g++-4.8-mp for instance.

OK. So what do you propose?