doc: add include statement into config reference

closes #2381

doc/reference.texi

@@ -12,6 +12,7 @@ This reference describes every configuration option in Knot DNS server.
 * control::
 * zones::                       
 * log::                         
+* include::
 @end menu
 
 @node system
@@ -982,3 +983,40 @@ log @{
 
 @end example
 
+@node include
+@section @code{include} Statement
+@stindex include
+
+The @code{include} statement is a special statement which can be used almost
+anywhere on any level in the configuration file. It makes inclusion of another
+file possible.
+
+The path of the included file can be either absolute or relative to a
+configuration file currently being processed.
+
+@menu
+* include Syntax::
+* include Examples::
+@end menu
+
+@node include Syntax
+@subsection Syntax
+
+@example
+@code{include} "filename"@code{;}
+@end example
+
+@node include Examples
+@subsection Examples
+
+@example
+include "keys.conf";
+
+remotes @{
+  ctl @{
+    address 127.0.0.1;
+    key knotc-key;
+  @}
+  include "remotes.conf";
+@}
+@end example