docs: Add some documentation and a README

docs/flow.md

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+
+# Message flows
+
+This document explains at a high level some parts of chasquid's message
+processing, in particular how messages flow through the system.
+
+
+## Message reception
+
+- Client connects to chasquid on the smtp or submission ports, and issues
+  HELO/EHLO.
+- Client optionally performs STARTTLS.
+- Client optionally performs AUTH.
+    - Check that this is done over TLS.
+- Client sends MAIL FROM.
+    - Check SPF.
+    - Check connection security level.
+- Client sends one or more RCPT TO.
+    - If the destination is remote, then the user must have authenticated.
+    - If the destination is local, check that the user exists.
+- Client sends DATA.
+- Client sends actual data, and ends it with '.'
+    - Run the post-data hook. If the hook fails, return an error.
+    - Parse the data contents to perform loop detection.
+    - Add the required headers (Received, SPF results, post-data hook output).
+    - Put it in the queue and reply success.
+
+
+## Queue processing
+
+Before accepting a message:
+
+- Create a (pseudo) random internal ID for it.
+- For each recipient, use the alias database to expand it, add the results to
+  the list of final recipients (which may not be email).
+- Save the resulting envelope (with the final recipients) to disk.
+
+Queue processing runs asynchronously, there's a goroutine for each message
+which does, in a loop:
+
+- For each recipient which we have not delivered yet:
+    - Attempt delivery.
+    - Write to disk the results.
+- If there are mails still pending, wait for some time (incrementally).
+- When all the recipients have completed delivery, or enough time has passed:
+    - If all were successful, remove from the queue.
+    - If some failed, send a delivery status notification back to the sender.
+