feat(secondary): add fallthrough support (#8041)

plugin/secondary/README.md

@@ -27,6 +27,7 @@ A working syntax would be:
 ~~~
 secondary [zones...] {
     transfer from ADDRESS [ADDRESS...]
+    fallthrough [ZONES...]
 }
 ~~~
 
@@ -34,6 +35,11 @@ secondary [zones...] {
    times; if one does not work, another will be tried. Transferring this zone outwards again can be
    done by enabling the *transfer* plugin.
 
+*  `fallthrough` If a query for a record in the zone results in NXDOMAIN, the query will be passed
+   to the next plugin in the chain. If **[ZONES...]** are listed, then only queries for those zones
+   will be subject to fallthrough. This can be useful in split DNS setups where the secondary zone
+   contains only partial records.
+
 When a zone is due to be refreshed (refresh timer fires) a random jitter of 5 seconds is applied,
 before fetching. In the case of retry this will be 2 seconds. If there are any errors during the
 transfer in, the transfer fails; this will be logged.