Expand the description of the RequireMetadata PG::Critic policy.

Author: somiaj

lib/Perl/Critic/Policy/PG/RequireMetadata.pm

@@ -48,4 +48,15 @@ metadata attributes should be set at the beginning of the problem file before
 the C<DOCUMENT> call.  The metadata tags that are required for all problems are
 C<DBsubject>, C<DBchapter>, C<DBsection>, and C<KEYWORDS>.
 
+The C<DBsubject>, C<DBchapter>, C<DBsection> tags can only be specific values.
+The prefered way to edit these tags is to use the tag editor (click "Edit Tags"
+when viewing the problem). The tag editor provides drop down menus to choose
+between the valid possibilities.
+
+Since OPL metadata tags are only needed for problems in the OPL, for local
+problems it is possible to override this check by adding the following as
+the first line of the PG file.
+
+    ## no critic (PG::RequireMetadata)
+
 =cut