It was a long time in coming, but Google Photos now can add text right to a photo. At least, the Android version of Google Photos can do that. It's called the "Markup" tool and it's pretty simple. If you have iPhone, we recommend the free app called Snapseed. See our YouTube show episode 136. Or take our Snapseed course.
Even if you know how to add text to photos with Google Photos, you should watch this short video because I add a very important little tip!
Notice how the words in the picture have a dark background. That is SO important to allow the words to be readable. I've found a way to accomplish it with the new Google Photos markup tool. Watch this video, it's less than 3 minutes.