Add changelog for 3.21.1

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

CHANGELOG.md

@@ -1,3 +1,11 @@
+## 3.21.1 (August 26, 2022)
+
+- Bug fixes
+
+Minimum: NC 16 Server, Android 6.0 Marshmallow
+
+For a full list, please see https://github.com/nextcloud/android/milestone/72
+
 ## 3.21.0 (August 8, 2022)
 
 - Fast scroll for all lists