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

Revert "Update to Gradle 4.1"

This reverts commit a54c18e52ed0e37b2e808249c61c1329d39dd517.

Restructure exercise directories

Reference solution directory was renamed and move to the
always-excluded-from-delivery .meta folder.

rna-transcription: update to v1.0.1 tests

Fixes #364 construct bob in setUp

Double indent code blocks to resolve list numbering

Add blurb to config.json

Fix track ordering

Side exercises can only be unlocked by core exercises. Previously we
had exercises being unlocked by both core and side exercises.

Each core exercise should unlock at least one exercise. Previously we
had some core exercises that didn't unlock any side exercises. Now
that's only true of the last core exercise in the track.