output github md instead of html

Author: grlloyd

vignettes/.gitignore

@@ -1 +1,2 @@
 *.R
+*.html

vignettes/struct_model.Rmd

@@ -1,6 +1,9 @@
 ---
 title: StRUCT Model Template
-output: rmarkdown::html_vignette
+output:
+    github_document:
+        df_print: paged
+        html_preview: false
 vignette: >
     %\VignetteIndexEntry{StRUCT Model Template}
     %\VignetteEngine{knitr::rmarkdown}