Add document and diagram for artifact movement

docs/movement.md

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+# How to move an artifact signed by tuf-notary
+
+<img src="images/Notary-v2_movement.jpg">
+
+The following is the process for moving an artifact that has been signed with TUF.
+We assume that both the source and destination registries have existing TUF metadata.
+
+1. Copy the artifact and any delegated targets metadata to the target repository on the destination registry.
+1. Update the top-level targets metadata on the destination repository to indicate that the original uploader's signature should be trusted
+1. (Optional) Add a signature to the artifact from the verifier or other entity and update the top-level targets metadata accordingly
+1. (Automated) Update the snapshot and timestamp metadata on the destination registry.
+
+
+All of these steps can be combined into a single CLI/API call with flags for adding signatures and indicating which keys should be trusted.