Added application certificate fingerprint to README

Signed-off-by: Dany Mestas <94061157+DanyPM@users.noreply.github.com>

README.md

@@ -13,6 +13,11 @@ height="80">](https://play.google.com/store/apps/details?id=com.nextcloud.client
 alt="Get it on F-Droid"
 height="80">](https://f-droid.org/packages/com.nextcloud.client/)
 
+Signing certificate fingerprint to [verify](https://developer.android.com/studio/command-line/apksigner#usage-verify) the APK:
+```
+SHA-256: fb009522f65e25802261b67b10a45fd70e610031976f40b28a649e152ded0373   
+```
+
 **The Android client for [Nextcloud](https://nextcloud.com). Easily work with your data on your Nextcloud.**
 
 ![App screenshots](/doc/Nextcloud_Android_Screenshots.png "App screenshots")