Add changelog for 3.18.0

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

CHANGELOG.md

@@ -1,3 +1,14 @@
+## 3.18.0 (November 18, 2021)
+
+- Calendar backup/restore
+- Unified search
+- Sharing permissions improvements
+- Minor bug fixes
+
+Minimum: NC 16 Server, Android 6.0 Marshmallow
+
+For a full list, please see https://github.com/nextcloud/android/milestone/61
+
 ## 3.17.1 (October, 20, 2021)
 
 - fix FIDO crash