Exclude secret deployment key in rsconnect/

Exclude secret deployment key in rsconnect/, see https://shiny.rstudio.com/articles/shinyapps.html for details.

R.gitignore

@@ -31,3 +31,6 @@ vignettes/*.pdf
 # Temporary files created by R markdown
 *.utf8.md
 *.knit.md
+
+# Shiny token, see https://shiny.rstudio.com/articles/shinyapps.html
+rsconnect/