Add missing headings

import.Rmd

@@ -3,7 +3,9 @@ layout: default
 title: Data import
 output: bookdown::html_chapter
 ---
-  
+
+# Data import
+
 ## Overview
 
 You can't apply any of the tools you've applied so far to your own work, unless you can get your own data into R. In this chapter, you'll learn how to import:

intro.Rmd

@@ -4,6 +4,8 @@ title: Welcome
 output: bookdown::html_chapter
 ---
   
+# Welcome
+  
 ## Overview
 
 The goal of "R for Data Science" is to give you a solid foundation into using R to do data science. The goal is not to be exhaustive, but to instead focus on what we think are the critical skills for data science:

transform.Rmd

@@ -4,6 +4,7 @@ title: Data transformation
 output: bookdown::html_chapter
 ---
 
+# Data transformation
 
 ## Missing values