docs: Add information about API to appinfo

Signed-off-by: Ferdinand Thiessen <rpm@fthiessen.de>

appinfo/info.xml

@@ -9,6 +9,7 @@
 - **📝 Simple design:** No mass of options, only the essentials. Works well on mobile of course.
 - **📊 View & export results:** Results are visualized and can also be exported as CSV in the same format used by Google Forms.
 - **🔒 Data under your control!** Unlike in Google Forms, Typeform, Doodle and others, the survey info and responses are kept private on your instance.
+- **🧑‍💻 Connect to your software:** Easily integrate Forms into your service with our full-fledged [REST-API](https://github.com/nextcloud/forms/blob/main/docs/API.md).
 - **🙋 Get involved!** We have lots of stuff planned like more question types, collaboration on forms, [and much more](https://github.com/nextcloud/forms/milestones)!
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