Format invariants as reST.

def _format_invariants(invariants: List[icontract._Contract]) -> List[str]:
    """Format invariants as reST."""
    if not invariants:
        return []

    result = [":establishes:"]  # type: List[str]
    for invariant in invariants:
        result.append("    * {}".format(_format_contract(contract=invariant)))

    return result