Add example on how to join list of strings

[MichaelAnckaert]

This PR includes an example on how to join a list of strings to a string, delimited with a character.

* (join '("foo" "bar" "baz") #\-)
"foo-bar-baz"

[pnathan]

consider using FORMAT as per either mine or R. Joswig's answer here: https://stackoverflow.com/questions/8830888/whats-the-canonical-way-to-join-strings-in-a-list

[vindarel]

Hi, thanks for sending a PR. I wish we could use shorter examples, especially when we have str:join or format.