added topics for difference-of-squares

Update nucleotide-count topics as in #808

Add topics for largest-series-product

Added @FridaTveit suggestions to nucleotide-count topics

* Updated Food-Chain Topics :)

* Removing plus signs from tags for food-chain

* minus comma

Added four tags to nucleotide-count topics as described in #808.

Add topics for acronym

Add topics to beer-song exercise

Add topics for space-age

Adding the list of topics for the phone-number problem

Add topics to pangram exercise

Add topics to twelve-days exercise

Adding the prerequisites to the CONTRIBUTING.md file

Add topics to raindrops exercise

* issue fixed

* minor bug fixed

* issue fixed #789

* minor bug fixed

Add Isogram topics

saddle-points: implement toString for matrix coordinate

Referencing #788

conditionals: finding if a char is repeated
loops: looping through a string's characters
parsing: parsing a string for repeated characters
strings: self-explanatory :)

referencing #788

conditionals: finding if a char is repeated
loops: looping through a string's characters
parsing: parsing a string for repeated characters
strings: self-explanatory :)

Added topics for Bob exercise

#753 Improve test names - difference-of-squares

gigasecond: add topics

This is to improve the error message when comparison fails.

hamming: add topics

two-fer: add topics

Fix mistake in contributing guide

Add topics for hello-world