add text to lockscreen

There isn't any built in way to do that. I did it using the following app: https://itunes.apple.com/us/app/if-found-lock-screen/id398340788?mt=8

Another option would be to use photo editing tools on computer and then transfer the edited photo to the phone.

First you need an app that will add text to a photo. Search the app store for an app that will do this. Then use that app, select a photo and add the appropriate text. Save the modified photo to your Photos app.

Then use the Settings app on your iPhone to select the modified photo to use as Wallpaper.

Settings > Wallpaper > Choose a New Wallpaper > Select the photo you saved to the Photos App