add article description

Author: ctoney

_pkgdown.yml

@@ -35,4 +35,5 @@ articles:
 external-articles:
 - name: stem-map-model
   title: Stem-map model manuscript (pdf)
+  description: A stem-map model for predicting tree canopy cover of Forest Inventory and Analysis (FIA) plots
   href: https://www.fs.usda.gov/rm/pubs/rmrs_p056/rmrs_p056_53_toney.pdf