plugin/file: add small zonefile example (#4509)

Watching TGI Kubernetes 147 and it helps to show a small zonefile
example in the documentation.
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/file/README.md

@@ -7,9 +7,9 @@
 ## Description
 
 The *file* plugin is used for an "old-style" DNS server. It serves from a preloaded file that exists
-on disk. If the zone file contains signatures (i.e., is signed using DNSSEC), correct DNSSEC answers
-are returned. Only NSEC is supported! If you use this setup *you* are responsible for re-signing the
-zonefile.
+on disk contained RFC 1035 styled data. If the zone file contains signatures (i.e., is signed using
+DNSSEC), correct DNSSEC answers are returned. Only NSEC is supported! If you use this setup *you*
+are responsible for re-signing the zonefile.
 
 ## Syntax
 
@@ -38,18 +38,32 @@ If you need outgoing zone transfers, take a look at the *transfer* plugin.
 
 ## Examples
 
-Load the `example.org` zone from `example.org.signed` and allow transfers to the internet, but send
+Load the `example.org` zone from `db.example.org` and allow transfers to the internet, but send
 notifies to 10.240.1.1
 
 ~~~ corefile
 example.org {
-    file example.org.signed
+    file db.example.org
     transfer {
         to * 10.240.1.1
     }
 }
 ~~~
 
+Where `db.example.org` would contain RRSets (<https://tools.ietf.org/html/rfc7719#section-4>) in the
+(text) presentation format from RFC 1035:
+
+~~~
+$ORIGIN example.org.
+@	3600 IN	SOA sns.dns.icann.org. noc.dns.icann.org. 2017042745 7200 3600 1209600 3600
+	3600 IN NS a.iana-servers.net.
+	3600 IN NS b.iana-servers.net.
+
+www     IN A     127.0.0.1
+        IN AAAA  ::1
+~~~
+
+
 Or use a single zone file for multiple zones:
 
 ~~~ corefile
@@ -92,4 +106,7 @@ example.org {
 ## See Also
 
 See the *loadbalance* plugin if you need simple record shuffling. And the *transfer* plugin for zone
-transfers. Lastly the *root* plugin can help you specificy the location of the zone files.
+transfers. Lastly the *root* plugin can help you specify the location of the zone files.
+
+See [RFC 1035](https://www.rfc-editor.org/rfc/rfc1035.txt) for more info on how to structure zone
+files.