How to add table of contents in Rmarkdown?

The syntax is

---
title: "Sample Document"
output:
  html_document:
    toc: true
    theme: united
---

in the documentation. Make sure this is at the beginning of your document. Also make sure your document actually has headers otherwise R can't tell what you want in the table of contents.

Syntax with more options:

---
title: "Planets"
author: "Manoj Kumar"
date: "`r format(Sys.time(), '%B %d, %Y')`"
output: 
  html_document:
    toc: true # table of content true
    toc_depth: 3  # upto three depths of headings (specified by #, ## and ###)
    number_sections: true  ## if you want number sections at each table header
    theme: united  # many options for theme, this one is my favorite.
    highlight: tango  # specifies the syntax highlighting style
    css: my.css   # you can add your custom css, should be in same folder
---