created extra_challenges.md

Following @maxim-belkin suggestion in #334, this file would store all extra challenges (exercises) that may be useful for instructors in the future.

Author: NPalopoli

_extras/extra_challenges.md

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+---   
+layout: page                                                                                            
+title: "Additional Exercises"                                                                           
+permalink: /extra_exercises/                                                                            
+---                                                                                                     
+
+# Extra Challenges
+
+A collection of challenges that have been either removed from or not (yet) added to the main lesson. 
+
+## 06-loops-and-functions
+
+### Looping Over Dataframe Challenges
+
+* What happens if there is no data for a year in the sequence (for example, imagine we had used 1976 as the start year in `range`)
+~~~
+An empty file with only the headers
+~~~
+{: .output}