Vignette could explain how to turn facebook data into CWB corpus

Then this snippet of a potential `as.xml()` function might be useful:

```r
sprintf(
  '<page party="%s" url="%s" date="%s">\n%s\n</page>',
  tbl$`User Name`, tbl$URL, tbl$date, tbl$Message
) %>%
  paste(collapse = "\n") %>%
  sprintf('<?xml version="1.0" encoding="utf-8"?>\n<collection>%s\n</collection>\n', .) %>%
  read_xml()
```