add msmtp docs (team#30608)

howto/submission.md

@@ -62,6 +62,24 @@ says that Access Now! has some docs we could start from. vegas agrees
 that this documentation and support will be covered by teams outside
 of TPA.
 
+## msmtp configuration
+
+This is how dgoulet configured his [msmtp client](https://marlam.de/msmtp/):
+
+    # Defaults for all accounts.
+    defaults
+    auth on
+    protocol smtp
+    tls on
+    port 587
+
+    # Account: dgoulet@torproject.org
+    account torproject
+    host submission.torproject.org
+    from dgoulet@torproject.org
+    user dgoulet
+    passwordeval pass mail/dgoulet@torproject.org
+
 # How-to
 
 <!-- more in-depth procedure that may require interpretation -->