Update install information

- using brew on mac OXS
- avoid redundant info between README and MANUAL
Not the same path:

```
$ g++-mp-4.8 -print-libgcc-file-name
/opt/local/lib/gcc48/gcc/x86_64-apple-darwin13/4.8.2/libgcc.a
```
vs
```
$ g++-mp-4.8 -print-file-name=libgomp.dylib
/opt/local/lib/gcc48/gcc/x86_64-apple-darwin13/4.8.2/../../../libgomp.dylib
```