initial fastlane structures from IzzyOnDroid

this PR provides you with a "fastlane starter package" – to give it into your hands to define how your app is presented, and make it easy to keep descriptions & graphics in sync with your development. Some notes to sum them up in a place easy to find for you: * the [IzzyOnDroid Fastlane Documentation](https://gitlab.com/IzzyOnDroid/repo/-/wikis/Fastlane) can guide you with maintaining the structures here * take special care for limits, e.g. per-release changelogs can have max 500 chars, `full_description.txt` max 4,000 chars, graphics must have specific aspect ratios etc. * you can have "bigger graphics" here; the IoD updater takes care to resize/optimize them as needed * for the `full_description.txt` here I've used HTML compressed into a single line, to prevent the fdroid software converting each line break to a `<br>`. The tags used for this are supported by F-Droid.org as well, and to my knowledge even by PlayStore. * whenever you add a new "type" of metadata that has not been there before (e.g. the first changelog, a featureGraphic if it wasn't yet present), please let us know so we can check and enable it. * Details on all this in the linked documentation. * Once merged, your changes will be synced whenever a new release is pulled in here – to make sure it stays up-to-date with the app itself. * Should you have questions, be welcome to ask :wink: With a separate commit, I've added the RB shield to your Readme – as your app was confirmed to be reproducible, see e.g. [Reproducible Builds, special client support and more at IzzyOnDroid](https://android.izzysoft.de/articles/named/iod-rbs-mirrors-clients) for some background. And now: Enjoy!