Q: can you add text to a photo

If you mean over the phone, then no, not directly. You can export the image and add it in Preview, or a downloaded, specialized app. You can also buy a $2 utility that will add all the Preview tools to Photos. It's an extension called External Editors.

If you mean add a title/description to the image file, then Yes. Right-click on it, then select Get Info and edit the tags you want.

The ability to write text into a photo from Photos will be added to Photos with the next macOS Sierra.  Then the "Markup" tool can be called as a Photo Editing extensions from Photos. Right now you can call Markup only from Mail as an action when you are mailing a photo. In Mail you can already write directly into a photo and annotate it in several ways.

http://www.apple.com/macos/sierra-preview/

In El Capitan use the "External Editors for Photos" photo editing extension pointed out by Rysz.  It is a "must have" when working with Photos and the price moderate - is just 99 cents at the AppStore (in Germany).

Hi Léonie, do you know if such a functionality will also be available on the iOS version of Photos soon? Thanks

The iOS10 preview does not say much about Photos. but it looks like it will have similar new features macOS Photos.