interactive bots: Add FourSquare bot and documentation.

contrib_bots/lib/FourSquareBot/doc.md

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+# FourSquare Bot
+
+* This is a bot that returns a list of restaurants from a user input of location,
+proximity and restaurant type in that exact order. The number of returned
+restaurants are capped at 3 per request.
+
+* The list of restaurants are brought to Zulip using an API. The bot sends a GET
+request to https://api.foursquare.com/v2/. If the user does not correctly input
+a location, proximity and a restaurant type, the bot will return an error message.
+
+* For example, if the user says "@foursquare 'Chicago, IL' 80000 seafood", the bot
+will return:
+
+Food nearby 'Chicago, IL' coming right up:
+
+    Dee's Seafood Co.
+    2723 S Poplar Ave, Chicago, IL 60608, United States
+    Fish Markets
+
+    Seafood Harbor
+    2131 S Archer Ave (at Wentworth Ave), Chicago, IL 60616, United States
+    Seafood Restaurants
+
+    Joe's Seafood, Prime Steak & Stone Crab
+    60 E Grand Ave (at N Rush St), Chicago, IL 60611, United States
+    Seafood Restaurants
+
+* If the user enters a wrong word, like "@foursquare 80000 donuts" or "@foursquare",
+then an error message saying invalid input will be displayed.
+
+* To get the required API Key, visit: https://developer.foursquare.com/overview/auth
+for more information.

contrib_bots/lib/FourSquareBot/settings.ini

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+[Foursquare]
+api_key = abcdefghijksm