Add tags in the `fastText`` format.
This format is require for the training step. As fastText doesn't support newlines inside documents
(as newlines are delimiting documents) this function also ensures that there are absolutely no
new lines. By default new lines are replaced by a single space.
# NOT RUN {library(fastrtext)
tags <- list(c(1, 5), 0)
documents <- c("this is a text", "this is another document")
add_tags(documents = documents, tags = tags)
# }