plugin/secondary: doc updates (#4686)

Add retry behavior on startup and point to RFC 5936
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/secondary/README.md

@@ -10,6 +10,9 @@ With *secondary* you can transfer (via AXFR) a zone from another server. The ret
 *not committed* to disk (a violation of the RFC). This means restarting CoreDNS will cause it to
 retrieve all secondary zones.
 
+If the primary server(s) don't respond when CoreDNS is starting up, the AXFR will be retried
+indefinitely every 10s.
+
 ## Syntax
 
 ~~~
@@ -67,3 +70,4 @@ Only AXFR is supported and the retrieved zone is not committed to disk.
 ## See Also
 
 See the *transfer* plugin to enable zone transfers _to_ other servers.
+And RFC 5936 detailing the AXFR protocol.