adding BUILD.md instructions, aggregated from Csound WIKI by Dominic Melville
adding BUILD.md instructions, aggregated from Csound WIKI by Dominic Melville

ketchupok/csound,csound/csound,csound/csound,ketchupok/csound,ketchupok/csound,csound/csound,csound/csound,ketchupok/csound,ketchupok/csound,csound/csound,ketchupok/csound,csound/csound,csound/csound,csound/csound,ketchupok/csound,csound/csound,ketchupok/csound,ketchupok/csound,csound/csound,ketchupok/csound