Correct faq entry on contact transfer

As mentioned correctly by @grote in
604c4e5f (comment 39593).

README.md

@@ -21,12 +21,24 @@ In case you forgot your password, there's no other way than to delete your old
 account and start from scratch. Call `rm -rf ~/.briar/` to delete your current
 account.
 
-### How can I transfer contacts from my Android device?
+### How can I transfer a Briar account from my Android device?
 
 By using [_adb_](https://developer.android.com/studio/command-line/adb),
 you can copy the database and keys from a Briar Android installation.
 Note that your smartphone needs to be rooted for this.
 
+You shouldn't use this technique to clone your Briar account to another device and
+then use the same account on both devices. If you do this, you open up yourself to many
+possible threats. See
+[this comment on Briar's issue tracker](https://code.briarproject.org/briar/briar/issues/110#note_34386)
+for more information. Therefore, make sure to delete the old copy
+of your Briar account after you've transferred it to another device to protect
+yourself against attacks circumventing forward secrecy.
+
+In case you just want to transfer contacts from one account to another,
+better use the "contact introduction feature" which is already available on Android
+devices and which is planned to land in python-briar-wrapper, too.
+
 You can either use the script at 
 [tools/copy-db-from-android.sh](tools/copy-db-from-android.sh) or do it manually: