R Markdown Add Tag to Head of HTML Output

You can create a file with the meta tag and add using the following YAML option:

---
title: "mytitle"
output:
  html_document:
    includes:
       in_header: myheader.html
---

You could also create the myheader.html file on the fly in the setup chunck:

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE )
#libraries
require(ggplot2)

#Create myheader.html
fileConn <- file("myheader.html")
writeLines('<meta http-equiv="X-UA-Compatible" content="IE=edge" />', fileConn)
close(fileConn)
```

Use library(metathis) !

Then include with:

```{r include=F}
meta() %>%
  meta_description("My awesome App")
```

And any other meta tag