Update BUILDING.rst
Update BUILDING.rst

Split sentences, improve build time estimate.

lethosor/Dwarf-Therapist,fricy/Dwarf-Therapist,lethosor/Dwarf-Therapist,lethosor/Dwarf-Therapist,jarmar/Dwarf-Therapist,fricy/Dwarf-Therapist,splintermind/Dwarf-Therapist,jarmar/Dwarf-Therapist,splintermind/Dwarf-Therapist,fricy/Dwarf-Therapist,lethosor/Dwarf-Therapist,splintermind/Dwarf-Therapist,jarmar/Dwarf-Therapist,fricy/Dwarf-Therapist,fricy/Dwarf-Therapist,jarmar/Dwarf-Therapist,splintermind/Dwarf-Therapist