Document or automate fig.showtext = TRUE

[Garrick Aden-Buie]

It might be strictly required that users include

```{r xaringan-themer, include = FALSE}
knitr::opts_chunk$set(fig.showtext = TRUE)
# ... style_* function()
```

for the font matching features of theme_xaringan() to work. If so, this needs to be prominently documented.

Alternatively, if it can be worked around and set automatically, that would be even better.

[Garrick Aden-Buie]

Closed in 1e6e514