openssl.pod: Add documentation for using the loader_attic engine
@@ -615,6 +615,12 @@ L<config(5)/Engine Configuration>.
The engine will be used for key ids specified with B<-key> and similar
options when an option like B<-keyform engine> is given.
+A special case is the C<loader_attic> engine, which
+is meant just for internal OpenSSL testing purposes and
+supports loading keys, parameters, certificates, and CRLs from files.
+When this engine is used, files with such credentials are read via this engine.
+Using the C<file:> schema is optional; a plain file (path) name will do.
+
=back
Options specifying keys, like B<-key> and similar, can use the generic
