Add non-default ssh key instructions to README

README

@@ -20,12 +20,13 @@ this by default, just say'in.
 Note: Knot handles DNSSEC signing on the server side.
 
 
-Using a non-default GPG key
----------------------------
+Using a non-default GPG/ssh key
+-------------------------------
 
-Create a file `local.mk` and specify GPG_FLAGS as follows:
+Create a file `local.mk` and specify GPG_FLAGS and/or SFTP_FLAGS as follows:
 
 ```
 # local.mk
 GPG_FLAGS=--default-key=<YOUR KEY FINGERPRINT>
+SFTP_FLAGS=~/.ssh/id_non_default
 ```