How to cite online-first published articles? Should you include link to earlier ArXiV version?
Here is an article about citing "advance online publication" in APA style.

Once an article is in the form of advanced online access at the publisher, you should generally cite this version, and not the ArXiv version. The online access is the final version of record. Page and volume numbers are cosmetic. 

The exact form of the reference will depend on your citation style. But I agree that including the doi in the reference is helpful as this will not change. In contrast, the year of the reference may change.

Here is an example from the post above:


  Muldoon, K., Towse, J., Simms, V., Perra, O., &amp; Menzies, V. (2012). A
           longitudinal analysis of estimation, counting skills, and 
           mathematical ability across the first school year. Developmental 
           Psychology. Advance online publication. doi:10.1037/a0028240


So in general the reference really shows almost no difference to a standard reference. It's just that the volume and page numbers are missing. You might append some text to indicate that it is "advance online publication", but this is optional in many cases. And you should probably include the doi (either in doi:10.. form, or in http://dx.doi.org/10... form). The doi will be helpful especially where the paginated version comes out in a subsequent year.

Obviously, depending on your citation style, you'd need to adapt the above. But the basic concept is the same: Authors, year, title, journal, doi, optionally some indication that it is an advance online publication. Basically, follow your normal citation strategy but omit the volume and page details and include a doi.