Added IzzeOnDroid as download location

Refs #215

README.md

@@ -120,6 +120,7 @@ Supported download locations:
 * [Play store](https://play.google.com/store/apps/details?id=eu.faircode.email)
 * [Play store](https://play.google.com/apps/testing/eu.faircode.email) (test)
 * [F-Droid](https://f-droid.org/en/packages/eu.faircode.email/) (the F-Droid app can be downloaded [here](https://f-droid.org/))
+* [IzzyOnDroid](https://android.izzysoft.de/repo/apk/eu.faircode.email) (verified GitHub release)
 * ~~[AppGallery](https://wap3.hispace.hicloud.com/uowap/index.jsp#/detailApp/C101678151) (the AppGallery app can be downloaded [here](https://huaweimobileservices.com/appgallery/))~~
 * ~~[Amazon](https://www.amazon.com/gp/product/B0983R6MH2)~~ (the APK file repackaged by Amazon is incomplete! An issue report was never answered by Amazon.)