update issue template, thanks @IzzySoft

.github/ISSUE_TEMPLATE.md

@@ -3,6 +3,12 @@
 - [ ] I have searched the [issues](https://github.com/Freeyourgadget/Gadgetbridge/issues), and I didn't find a solution to my problem / an answer to my question.
 - [ ] If you upload an image or other content, please make sure you have read and understood the [github policies and terms of services](https://help.github.com/articles/github-terms-of-service/#1-responsibility-for-user-generated-content)
 
+### I got Gadgetbridge from:
+* [ ] F-Droid
+* [ ] I built it myself from source code (specify tag / commit)
+
+If you got it from Google Play, please note [that version](https://github.com/TaaviE/Gadgetbridge) is unofficial and not supported here; it's also often quite outdated. Please switch to one of the above versions if you can.
+
 #### Your issue is:
 *In case of a bug, do not forget to attach logs!*