Document how to generate SSL cert fingerprint

electrum/commands.py

@@ -1263,7 +1263,8 @@ argparse._SubParsersAction.__call__ = subparser_call
 
 
 def add_network_options(parser):
-    parser.add_argument("-f", "--serverfingerprint", dest="serverfingerprint", default=None, help="only allow connecting to servers with a matching SSL certificate SHA256 fingerprint")
+    parser.add_argument("-f", "--serverfingerprint", dest="serverfingerprint", default=None, help="only allow connecting to servers with a matching SSL certificate SHA256 fingerprint." + " " +
+                                                                                                  "To calculate this yourself: '$ openssl x509 -noout -fingerprint -sha256 -inform pem -in mycertfile.crt'. Enter as 64 hex chars.")
     parser.add_argument("-1", "--oneserver", action="store_true", dest="oneserver", default=None, help="connect to one server only")
     parser.add_argument("-s", "--server", dest="server", default=None, help="set server host:port:protocol, where protocol is either t (tcp) or s (ssl)")
     parser.add_argument("-p", "--proxy", dest="proxy", default=None, help="set proxy [type:]host[:port] (or 'none' to disable proxy), where type is socks4,socks5 or http")