* Add hints.md to armstrong-numbers

* Regenerate armstrong-numbers README

* Update bracket-push tests to match canonical data

* Update bracket-push version file

* go-counting: use provided enumeration instead of Strings

Use of enumeration should be consistent across the test suite.

* update example solution to match new test expectations

* fix return type

robot-simulator: remove `final` from tests #1453

poker: rename version file #1454

rail-fence-cipher: add version file (1.1.0) #1405

Removed `final` from tests

go counting: remove .keep

regenerate READMEs

Merge

* Poker: Update test to exactly match the canonical data and add version file

* Change PokerTest.java,Card.java and Hand.java in order to match with canonical data and add version file.

* Correct Card.java in order to match with canonical data.

* Change indentation and correct mistakes

* Add import to PokerTest

* Fix indentation and correct mistakes

* Change variable's name from snake case to camel case

* Fix variable's name

binary-search-tree: update tests and add version file #1408

* added hints.md to .meta

* generated README and removed hint header

* Remove final keyword from variables

* Remove final keyword from tests

* grep: WIP add exercise

* grep: implement exercise

* grep: add to config.json

* grep: update config.json and add hints

* grep: Add .keep file

* grep: improve reference solution

* grep: Make sure settings.gradle is in alphabetical order

poker: remove `final` from tests #1426

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.