added dh-params option into sample file

doc/sample.config

@@ -50,6 +50,12 @@ try-mtu-discovery = false
 server-cert = ../test-cert.pem
 server-key = ../test-key.pem
 
+# Diffie-Hellman parameters. Only needed if you require support
+# for the DHE ciphersuites (by default this server supports ECDHE).
+# Can be generated using:
+# certtool --generate-dh-params --outfile /path/to/dh.pem
+dh-params = dh.pem
+
 # In case PKCS #11 or TPM keys are used the PINs should be available
 # in files. The srk-pin-file is applicable to TPM keys only (It's the storage
 # root key).