Document -engine where missing.
PR: 424

doc/apps/spkac.pod

diff --git a/doc/apps/spkac.pod b/doc/apps/spkac.pod
index bb84dfb..c3f1ff9 100644
--- a/doc/apps/spkac.pod
+++ b/doc/apps/spkac.pod
@@ -17,7 +17,7 @@
 [B<-spksect section>]
 [B<-noout>]
 [B<-verify>]
-
+[B<-engine id>]
 
 =head1 DESCRIPTION
 
@@ -79,6 +79,12 @@
 
 verifies the digital signature on the supplied SPKAC.
 
+=item B<-engine id>
+
+specifying an engine (by it's unique B<id> string) will cause B<req>
+to attempt to obtain a functional reference to the specified engine,
+thus initialising it if needed. The engine will then be set as the default
+for all available algorithms.
 
 =back