Remove broken link from contributing guide

Update policies

Add updating readmes and checking tests sections to contributing guide

Add some more best practices that I've seen come up on reviews quite often.

Add a section on not returning null since the canonical data often cause
confusion about this.

Add a section on using ExpectedException since this was something we agreed
a while ago but it's not documented anywhere.

Add reference tutorial policy

We have decided that every exercise with diffulty less than 3 should
reference the hello world tutorial to make it easier for people to solve
those exercises. This has now been added to our policies doc.

We also now reference those policies in the contributing doc in the section
for adding new exercises.

update readmes

Fix canonical data check script

The script was reporting version as different because of windows line
ending. These line endings are now being removed before the version
comparison.

Update contributing.md

forth: update version file (1.4.0 -> 1.5.0) #1377

Make link to policies more general so it doesn't need updating every
time the policies change.

Add hint to only fix one issue per PR and do all changes on a branch.

change: update version file (1.1.0 -> 1.2.0) #1376
list-ops: update version file (2.0.0 -> 2.2.0) #1375
book-store: update version file (1.2.0 -> 1.3.0) #1373
circular-buffer: update version file (1.0.1 -> 1.1.0) #1371
ocr-numbers: update version file (1.0.0 -> 1.1.0) #1370
wordy: update version file (1.0.0 -> 1.1.0) #1362
minesweeper: update version file (1.0.0 -> 1.1.0) #1361

custom-set: update version file (1.1.0 -> 1.3.0) #1378

* update version file to 1.1.0

* add test to verify no rectangle when there are no rows

* fix test naming to match canonical data

* correct spacing for added test

* update house version to 2.2.0

* update reference solution to reflect canonical data

* sync house tests with canonical data

* two-bucket: increase difficulty from 5 to 7

* move position of two-bucket in config.json file

* dominoes: add new exercise

* dominoes: add new exercise

* dominoes: add new exercise

* dominoes: add new exercise

* dominoes: add new exercise

* dominoes: add new exercise

* dominoes: add new exercise

* dominoes: add new exercise

* update queen-attack tests to version 2.1.0

* remove unchecked exceptions signature

* Update version

robot-simulator version file update

* Update RobotTest.java

Have only checked against canonical data up to the commented line.

* Update RobotTest.java

Update test for bowling

protein-translation: update unlocked by

Update version for clock Challenge

It was previously unlocked by rna-transcription which is not a core exercise.
Exercises must be unlocked by core exercises.

Update version for markdown Challenge

Update crypto-square version File

* Update tests to match canonical data version 1.3.0

* Update version file (1.0.0 -> 1.3.0)

Update version for isogram Exercise

* go-counting: add new exercise

* go-counting: add new exercise

* go-counting: add new exercise

* go-counting: add new exercise

* go-counting: add exercise

* go-counting: add new exercise

Alphametics: Update and Move version File

Update version for perfect-numbers

* Add new exercise Variable Length Quantity

* Fix test file

* Get rid of unchecked/unsafe operations warning

* Fix typo in config.json

* Add List imoprt to starter solution

* Break up some tests into multiple lines

* Throw an IllegalArgumentException in error cases.

* Add more topics

* Indent multi-line arguments

* Add new exercise Error Handling

* Add requested changes

* Undo unnecessary change to ErrorHandlingTest.java

* Add requested changes

* Add requested changes

* Check exception message

* Add requested changes to ErrorHandlingTest.java

* Add hints.md

* Add requested suggestions

* Expand hints.md

* Add gramatical suggestions