SETUP_AND_SHUTDOWN process doc

docs/SETUP_AND_SHUTDOWN.md

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+# Setup and Shutdown
+
+This may change, but this is approximately the order of events.
+
+- bin::main()
+    - setup logging
+    - lib::init()
+        - storage::init()
+            - trigger database into existence
+            - migrate
+        - signer::init()
+            - load from settings in storage
+        - deletation init
+        - setup wait-for-login state
+    - setup async runtime
+    - optionally handle command-line command and exit, else
+    - spawn (two threads as below)
+
+- spawn-thread
+    - lib::run()
+        - overlord::run()
+            - maybe wait-for-login (the UI has to do it)
+            - start fetcher
+            - start People tasks
+            - start relay picker
+            - pick relays
+            - subscribe discover
+            - subscribe outbox
+            - subscribe inbox
+            - loop
+                - Get and handle messages
+                - or if shutdown variable is set, exit this loop
+            - storage::sync()
+            - set shutdown variable
+            - message minions to shutdown
+            - wait for minions to shutdown
+    - end of spawn-thread
+
+- main-thread
+    - ui::run()
+        - Setup and run the UI
+        - if wait-for-login, prompt for password and login
+        - once logged in, indicate such so the overlord can start, and run UI as normal
+        - If shutdown variable is set, exit
+    - Signal overlord to shutdown
+    - Wait for spawn-thread to end
+    - lib::shutdown()
+        - storage::sync()