How do I reference a constitution?
First thing: the exact format will depend on the styleguide your are following. However, apart from variations in the exact formatting, just refer to it exactly as you do. The US constitution, for example, would be cited in APA style as one of the following:


  
  U.S. Const. art. I, § 3.
  U.S. Const. amend. XIX.
  U.S. Const. amend. XVIII (repealed 1933).
  


depending on whether you cite an article from the original constitution, or a (possibly since-repealed) amendment.



Somewhat tongue-in-cheek edit: I could not find a DOI (Digital Object Identifier) for the constitution of Montenegro (nor for the US constitution, for that matter). If you find one, you might want to include it in your citation (in the form of a hyperlink). If it does not exist, you may petition the Montenegrin government to create one.