README: mention TLS

Author: guggero

README.md

@@ -333,6 +333,18 @@ pending swaps after a restart.
 Information about pending swaps is stored persistently in the swap database.
 Its location is `~/.loopd/<network>/loop.db`.
 
+## Transport security
+
+The gRPC and REST connections of `loopd` are encrypted with TLS the same way
+`lnd` is.
+
+If no custom loop directory is set then the TLS certificate is stored in
+`~/.loopd/<network>/tls.cert`.
+
+The `loop` command will pick up the file automatically on mainnet if no custom
+loop directory is used. For other networks it should be sufficient to add the
+`--network` flag to tell the CLI in what sub directory to look for the files.
+
 ## Multiple Simultaneous Swaps
 
 It is possible to execute multiple swaps simultaneously. Just keep loopd