Add bt-setup stuff

sandbox/bt-setup.md

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+# Use Cases
+
+## Assumptions about users
+
+Fair?
+
+- they know that Briar can only deliver a message when the other contact
+  appears as online
+
+- they have some vague concept of Bluetooth, and that BT is one way of
+  sending a message to a contact
+
+## Case: user A wants to send something to nearby contact B
+
+We don't have any Internet-connection or even Wifi, so we'll use this BT.
+
+- I open the conversation with contact B,
+  - to send a message
+  - make an introducton
+  - to see that the group invitation/forum sharing/etc that I made
+    has not been sent off (little clock icon visible)
+
+- We both need to have BT turned on! I turn BT on, and tell B to do the same.
+
+- I don't see B as online!
+  - Alternative: B already appears as online, so I can send something. DONE.
+
+- I tap the "Connect via Bluetooth" button in the conversation
+  (or understandable icon)
+
+- Screen comes up:
+  ```
+  Make all contacts nearby go to this screen now and press start roughly at the
+  same time
+
+  [Start]
+  ```
+  - Alternative: B now appears as online, and a toast highlights this fact to me
+
+- TODO: I tell B "how to go to this screen"
+
+- I tell B to press `Start`
+
+- Screen: indicates that something is in progress
+
+- Screen: shows that contact B has been discovered, `OK` button
+  - Alternative: screen also shows that my contact C has been discovered
+
+- I tap `OK`
+
+- I see that B appears as online. DONE.