By default, R Markdown displays data frame and matrix output as it would appear at the command line:

```{r echo = FALSE}
mtcars[1:5, ]
```

Format the output as a table with knitr's `kable` function:

```{r echo = FALSE}
library(knitr)
kable(mtcars[1:5, ], caption = "A knitr kable.")
```