docs: Add rough notes about moving accounts around.

docs/transfer_account.md

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+# How to Transfer an Account
+
+Secure Scuttlebutt accounts can be easily transferred between apps and devices.
+
+However, it is not recommended to use an account on multiple devices. If you
+author a message on one device without having received all messages authored
+from another, your account may become irrecoverably forked. Other clients may
+stop receiving your messages if this happens.
+
+1. In Tilde Friends, the _identity_ app will let you export and import your identity as a series of twelve English words for copying and pasting.  Keep these words secret!
+
+2. The _sneaker_ app will let you export and import your feed to a file, but it's likely easier and faster to use your initial account on the receiver as a throwaway account to connect, follow yourself, and do the initial replication.
+
+Your identity and messages on the target device is all you need to resume posting. Deleting the identity from the source to avoid accidentally using it is probably a wise idea.
+
+If you are moving accounts between applications on the same device, note that they may not be able to see each other if they attempt to use the same network port number.
+
+You may also wish to make your account the server identity in the _identity_ app so that other devices on the network see and connect to you by your well-known identity.