That for the purpose of erecting, repairing or improving library buildings or purchasing sites for library buildings, the directors of any township library, organized under the laws of this state, when thereunto authorized by majority of all the votes cast on the proposition at an election, may borrow money, and may issue bonds therefor, in the sums of not less than one hundred dollars ($100) bearing interest at a rate not exceeding six per centum per annum and for a term not to exceed twenty years: Provided, that the sum borrowed in any one year shall not exceed five per centum (including existing indebtedness) of the taxable property of the township, to be ascertained by the last assessment for state and county taxes previous to the incurring of such indebtedness. The proposition shall be submitted to the voters in accordance with the general election law upon the adoption of a resolution so requesting by the directors and certified to the proper election officials.