add unambiguous flag for Copernicus template detection (#240)

Author: nuest

inst/rmarkdown/templates/copernicus_article/resources/template.tex

@@ -5,6 +5,7 @@
 %% For further assistance please contact Copernicus Publications at: production@copernicus.org
 %% https://publications.copernicus.org/for_authors/manuscript_preparation.html
 
+%% copernicus_rticles_template (flag for rticles template detection - do not remove!)
 
 %% Please use the following documentclass and journal abbreviations for discussion papers and final revised papers.