zulip_bots: Add link to get API Key in Link Shortener Bot Documentation.
Link Shortener Bot is a Zulip bot that will shorten URLs ("links") in a
conversation. It uses the [goo.gl URL shortener API] to shorten its links.
+Use [this](https://developers.google.com/url-shortener/v1/getting_started) to get
+your API Key.
+
Links can be anywhere in the message, for example,
> @**Link Shortener Bot** @**Joe Smith** See
